Polycarboxylic acid mixture

ABSTRACT

A polycarboxylic acid mixture comprising 80% by weight or more of 1,3,6-hexanetricarboxylic acid, wherein the polycarboxylic acid mixture has a psychometric lightness L-value of 98 or more, a psychometric chroma a-value of from −2.0 to 2.0 and a psychometric chroma b-value of from −2.0 to 3.0, and has a nitrogen content of 5,000 ppm by weight or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polycarboxylic acid mixture. Moreparticularly, the present invention is concerned with a polycarboxylicacid mixture comprising 80% by weight or more of1,3,6-hexanetricarboxylic acid, wherein the polycarboxylic acid mixturehas a psychometric lightness L-value of 98 or more, a psychometricchroma a-value of from −2.0 to 2.0 and a psychometric chroma b-value offrom −2.0 to 3.0, and has a nitrogen content of 5,000 ppm by weight orless. The polycarboxylic acid mixture of the present invention has notonly excellent color tone (i.e., not discolored and substantiallycolorless and transparent) but also excellent color tone stability underheating (hereinafter, this stability is frequently referred to simply as“heat stability”). Therefore, the polycarboxylic acid mixture of thepresent invention can be advantageously used for producing, for example,a paint, a detergent, a builder for a cleaning agent, an anti-limescaleagent, a lubricating oil, and various polycarboxylic acid derivatives,such as esters. The present invention relates also to a method forproducing the above-mentioned polycarboxylic acid mixture in high yield.

2. Prior Art

1,3,6-Hexanetricarboxylic acid is water-soluble and has goodbiodegradability. Therefore, recently, various application fields of1,3,6-hexanetricarboxylic acid have been proposed.

For example, it has been reported that 1,3,6-hexanetricarboxylic acidwhich is derived from 1,3,6-tricyanohexane, as well as a salt of1,3,6-hexanetricarboxylic acid, can be advantageously used as a rawmaterial for preparing a detergent and as a lubricating agent for achain belt for a toothed wheel (see Unexamined German Patent ApplicationLaid-Open Specification No. 19637428).

1,3,6-Hexanetricarboxylic acid is a trifunctional acid; therefore, whenit is used as a curing agent for, e.g., an epoxy compound, a curedproduct having a high crosslinkage density can be obtained. Especially,1,3,6-hexanetricarboxylic acid can be advantageously used as acrosslinking agent for a paint.

Generally, 1,3,6-hexanetricarboxylic acid can be easily obtained byhydrolyzing 1,3,6-tricyanohexane. 1,3,6-Tricyanohexane can be obtainedas a by-product in a process for producing adiponitrile fromacrylonitrile by electrodimerization. 1,3,6-Tricyanohexane can also beproduced by reacting acrylonitrile with adiponitrile in the presence ofa base.

When an electrodimerization reaction of acrylonitrile is conducted,1,3,6-tricyanohexane is obtained as a by-product and contained in amixture of nitrites including adiponitrile as a main product. Byremoving the adiponitrile from the mixture by distillation or the like,the content of 1,3,6-tricyanohexane in the mixture can be increased.

However, the nitrile mixture comprised mainly of 1,3,6-tricyanohexane,which is obtained by the above-mentioned methods, is usually markedlydiscolored to assume a color of yellow or black.

For example, in Unexamined Japanese Patent Application Laid-OpenSpecification No. Sho 62-270550, 1,3,6-tricyanohexane is purified bymolecular distillation, but the resultant purified product is a yellowliquid having a Hazen value of 400 or more.

A polycarboxylic acid mixture comprised mainly of1,3,6-hexanetricarboxylic acid, which is conventionally obtained byhydrolyzing such a markedly discolored nitrile mixture as mentionedabove, is also markedly discolored to assume a color of, for example,yellow brown or red brown. Further, such a discolored polycarboxylicacid mixture has a problem in that, when the mixture is heated, forexample, at 80° C. or more, it exhibits an increased degree ofdiscoloration; specifically, the color difference (ΔE) as between beforeand after the heating tends to be far greater than 1 and, hence, thecolor tone stability of the discolored polycarboxylic acid mixture underheating is poor. Therefore, a number of attempts have been made toobtain a polycarboxylic acid mixture which has an improved color tone byremoving the undesired color from the discolored polycarboxylic acidmixture comprised mainly of 1,3,6-hexanetricarboxylic acid.

For example, Unexamined German Patent Application Laid-OpenSpecification No. 19637428 discloses the following method for producinga colorless or light yellow 1,3,6-hexanetricarboxylic acid. A nitrilemixture comprised mainly of 1,3,6-tricyanohexane, which is by-producedin an electrodimerization reaction of acrylonitrile, is hydrolyzed byheating, under reflux, the nitrile mixture together with a 20% aqueoussodium hydroxide solution to thereby obtain a hydrolysis reactionmixture, followed by cooling of the reaction mixture. A concentratedsulfuric acid is added to the obtained hydrolysis reaction mixture tothereby obtain a carboxylic acid mixture. The obtained carboxylic acidmixture is thoroughly dried to thereby obtain a beige residue. The beigeresidue is subjected to an extraction treatment with a dehydrated alkylacetate by means of a Soxhlet's extractor, to obtain an extract,followed by removal of the alkyl acetate from the obtained extract underreduced pressure, thereby obtaining a colorless or light yellow1,3,6-hexanetricarboxylic acid. Also, in this patent document, as amethod other than the above-mentioned method using a Soxhlet'sextractor, there is also described the following method for producing acolorless or light yellow 1,3,6-hexanetricarboxylic acid. Theabove-mentioned carboxylic acid mixture is extracted with tert-butylmethyl ether three times to thereby obtain an extract. Water is removedfrom the obtained extract by using magnesium sulfate, and the resultantis subjected to distilling off of the extraction solvent to therebyobtain a residue. The obtained residue is introduced into a mixture ofacetone and cyclohexane, and the resultant mixture is cooled to therebydeposit crystals, followed by recovery of the crystals by filtration,thereby obtaining a colorless or light yellow 1,3,6-hexanetricarboxylicacid.

Further, in the above-mentioned Unexamined German Patent ApplicationLaid-Open Specification No. 19637428, there is also described thefollowing method for producing a colorless or light yellow1,3,6-hexanetricarboxylic acid. 1,3,6-Tricyanohexane is mixed withcrushed ice and a concentrated sulfuric acid, and the resultant mixtureis subjected to hydrolysis at 140° C. to thereby obtain an aqueousmixture. The obtained aqueous mixture is extracted with tert-butylmethyl ether three times to thereby obtain an ether mixture. Water isremoved from the obtained ether mixture by using magnesium sulfate, andthe resultant is subjected to distilling off of the ether, therebyobtaining a colorless or light yellow 1,3,6-hexanetricarboxylic acid.

However, a 1,3,6-hexanetricarboxylic acid obtained by any of theabove-mentioned methods described in Unexamined German PatentApplication Laid-Open Specification No. 19637428 is disadvantageous inthat it has a psychometric lightness L-value of less than 98 or apsychometric b-value of 3 or more and has poor heat stability such thatthe color difference (ΔE) as between before and after the1,3,6-hexanetricarboxylic acid is allowed to stand for 18 hours at 80°C. is more than 2. (With respect to the above-mentioned colordifference, explanations are given below.) Further, in any of theabove-mentioned methods described in Unexamined German PatentApplication Laid-Open Specification No. 19637428, it is necessary to usea large amount of an extraction solvent and, therefore, these methodsare commercially disadvantageous.

Further, Agric. Biol. Chem., 45 (1) 57-62, 1981 discloses the followingmethod for producing a colorless crystal of 1,3,6-hexanetricarboxylicacid. 1,3,6-Tricyanohexane is hydrolyzed with hydrochloric acid, and theresultant hydrolysis reaction mixture is allowed to stand at roomtemperature to thereby precipitate an ammonium salt (ammonium chloride),followed by filtration of the resultant to remove the ammonium salt andobtain a filtrate. The obtained filtrate is dried to obtain a crystal of1,3,6-hexanetricarboxylic acid having a low purity. The obtained lowpurity crystal is dissolved in hot water, and the resultant solution istreated with activated carbon, followed by recrystallization, therebyobtaining a colorless crystal of 1,3,6-hexanetricarboxylic acid. Theobtained 1,3,6-hexanetricarboxylic acid has a psychometric lightnessL-value of 98 or more, a psychometric chroma a-value of 0.8 and apsychometric chroma b-value of 1.1, and is colorless. However,1,3,6-hexanetricaboxylic acid produced by this method exhibits poor heatstability such that, when the 1,3,6-hexanetricaboxylic acid is heated,for example, at 160° C. for one hour or more, it is likely to suffer amarked discoloration. Further, the 1,3,6-hexanetricaboxylic acidproduced by this method tends to disadvantageously contain a chlorineion.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies with a view toward developing a polycarboxylic acidmixture comprised mainly of 1,3,6-hexanetricaboxylic acid, wherein thepolycarboxylic acid mixture has not only a high level of color tonewhich is required in the field of paints, but also a high level of heatstability such that the color difference (ΔE) as between before andafter the polycarboxylic acid mixture is allowed to stand at 80° C. for18 hours is 1 or less, and the color difference (ΔE) as between beforeand after the polycarboxylic acid mixture is allowed to stand at 160° C.for 3 hours is 10 or less (the above-mentioned color difference isexplained below). As a result, it has surprisingly been found that suchexcellent properties can be exhibited by a polycarboxylic acid mixturecomprising 80% by weight or more of 1,3,6-hexanetricarboxylic acid,wherein the polycarboxylic acid mixture has a psychometric lightnessL-value of 98 or more, a psychometric chroma a-value of from −2.0 to 2.0and a psychometric chroma b-value of from −2.0 to 3.0, and has anitrogen content of 5,000 ppm by weight or less. Based on this novelfinding, the present invention has been completed.

Accordingly, the primary object of the present invention is to provide apolycarboxylic acid mixture which has not only excellent color tone, butalso an extremely high level of heat stability.

Another object of the present invention is to provide a method forproducing the above-mentioned excellent polycarboxylic acid mixtureeasily and efficiently from a hydrolysis reaction mixture obtained byhydrolyzing a nitrile mixture comprised mainly of 1,3,6-tricyanohexane.

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andappended claims.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect of the present invention, there is provided apolycarboxylic acid mixture comprising 80% by weight or more of1,3,6-hexanetricarboxylic acid, the polycarboxylic acid mixture having apsychometric lightness L-value of 98 or more, a psychometric chromaa-value of from −2.0 to 2.0 and a psychometric chroma b-value of from−2.0 to 3.0,

-   -   the polycarboxylic acid mixture having a nitrogen content of        5,000 ppm by weight or less.

In another aspect of the present invention, there is provided a methodfor producing the above-mentioned polycarboxylic acid mixture, whichcomprises the steps of:

-   -   (1) adjusting the pH value of an aqueous hydrolysis reaction        mixture solution obtained by hydrolyzing, in an aqueous medium,        a nitrile mixture comprised mainly of 1,3,6-tricyanohexane to a        level in the range of from 3 to 13, thereby obtaining an aqueous        solution containing a salt of a polycarboxylic acid mixture, the        nitrile mixture being obtained as a by-product in a process for        producing adiponitrile from acrylonitrile by electrodimerization        or obtained by reacting acrylonitrile with adiponitrile,    -   (2) treating the aqueous solution with a solid adsorbent to        obtain a treated aqueous solution,    -   (3) converting the salt of a polycarboxylic acid mixture in the        treated aqueous solution obtained in step (2) to a        polycarboxylic acid mixture using an ion exchange resin, an        electrodialyzer or an acid, thereby obtaining an aqueous        solution containing a polycarboxylic acid mixture, and    -   (4) recovering the polycarboxylic acid mixture from the aqueous        solution obtained in step (3),    -   wherein when the acid is used in step (3) for converting the        salt to the polycarboxylic acid mixture, the recovered        polycarboxylic acid mixture is subjected to an extraction with        an organic solvent for the polycarboxylic acid mixture to obtain        the polycarboxylic acid mixture as an extract with the organic        solvent, followed by separation of the polycarboxylic acid        mixture in the extract from the organic solvent.

For easy understanding of the present invention, the essential featuresand various preferred embodiments of the present invention areenumerated below.

1. A polycarboxylic acid mixture comprising 80% by weight or more of1,3,6-hexanetricarboxylic acid,

-   -   the polycarboxylic acid mixture having a psychometric lightness        L-value of 98 or more, a psychometric chroma a-value of from        −2.0 to 2.0 and a psychometric chroma b-value of from −2.0 to        3.0,    -   the polycarboxylic acid mixture having a nitrogen content of        5,000 ppm by weight or less.

2. The polycarboxylic acid mixture according to item 1 above, which hasa psychometric lightness L-value of 99 or more, a psychometric chromaa-value of from −1.0 to 1.0 and a psychometric chroma b-value of from−1.0 to 1.0.

3. The polycarboxylic acid mixture according to item 1 or 2 above, whichhas a nitrogen content of 500 ppm by weight or less.

4. The polycarboxylic acid mixture according to any one of items 1 to 3above, which is obtained from a hydrolysis reaction mixture obtained byhydrolyzing a nitrile mixture comprised mainly of 1,3,6-tricyanohexane,the nitrile mixture being obtained as a by-product in a process forproducing adiponitrile from acrylonitrile by electrodimerization orobtained by reacting acrylonitrile with adiponitrile.

5. A method for producing the polycarboxylic acid mixture of item 1above, which comprises the steps of:

-   -   (1) adjusting the pH value of an aqueous hydrolysis reaction        mixture solution obtained by hydrolyzing, in an aqueous medium,        a nitrile mixture comprised mainly of 1,3,6-tricyanohexane to a        level in the range of from 3 to 13, thereby obtaining an aqueous        solution containing a salt of a polycarboxylic acid mixture,    -   the nitrile mixture being obtained as a by-product in a process        for producing adiponitrile from acrylonitrile by        electrodimerization or obtained by reacting acrylonitrile with        adiponitrile,    -   (2) treating the aqueous solution with a solid adsorbent to        obtain a treated aqueous solution,    -   (3) converting the salt of a polycarboxylic acid mixture in the        treated aqueous solution obtained in step (2) to a        polycarboxylic acid mixture using an ion exchange resin, an        electrodialyzer or an acid, thereby obtaining an aqueous        solution containing a polycarboxylic acid mixture, and    -   (4) recovering the polycarboxylic acid mixture from the aqueous        solution obtained in step (3),    -   wherein when the acid is used in step (3) for converting the        salt to the polycarboxylic acid mixture, the recovered        polycarboxylic acid mixture is subjected to an extraction with        an organic solvent for the polycarboxylic acid mixture to obtain        the polycarboxylic acid mixture as an extract with the organic        solvent, followed by separation of the polycarboxylic acid        mixture in the extract from the organic solvent.

6. The method according to item 5 above, wherein the aqueous medium usedfor the hydrolyzing of the nitrile mixture in step (1) is water.

7. The method according to item 5 above, wherein, in step (1), the pHvalue of the aqueous solution is adjusted to a level in the range offrom 5 to 9.

8. The method according to item 5 above, wherein the solid adsorbentused in step (2) is at least one adsorbent selected from the groupconsisting of an activated carbon, a silica gel and an activatedalumina.

9. The method according to item 5 above, wherein the conversion of thesalt to the polycarboxylic acid mixture in step (3) is performed usingthe electrodialyzer.

10. The method according to any one of items 5 to 9 above, wherein theaqueous solution obtained in step (3) is subjected to crystallizationbefore step (4).

11. A curable composition comprising:

-   -   (a) a compound having two or more epoxy groups in a molecule        thereof, and    -   (b) a curing agent comprising the polycarboxylic acid mixture of        item 1 above.

12. A paint comprising the curable composition of item 11 above.

13. A cured composition obtained by curing the curable composition ofitem 11 above.

Hereinbelow, the present invention is described in detail.

The polycarboxylic acid mixture of the present invention comprises 80%by weight or more of 1,3,6-hexanetricarboxylic acid (i.e.,4-carboxy-1,8-octanedioic acid). With respect to the content of apolycarboxylic acid other than 1,3,6-hexanetricarboxylic acid in thepolycarboxylic acid mixture, there is no particular limitation. In thepresent invention, the term “polycarboxylic acid” means an aliphaticcarboxylic acid having 2 to 4 carboxyl groups in a molecule thereof andhaving a molecular weight of 400 or less. Specific examples ofpolycarboxylic acids other than 1,3,6-hexanetricarboxylic acid includeadipic acid and 3-carboxymethyl-1,5-pentanedicarboxylic acid.

With respect to the composition of the polycarboxylic acid mixture ofthe present invention, there is no particular limitation so long as thecontent of 1,3,6-hexanetricarboxylic acid in the polycarboxylic acidmixture is 80% by weight or more. However, taking into consideration thecase where in the production of a paint by using the polycarboxylic acidmixture of the present invention, the polycarboxylic acid mixture isdissolved in a solvent at about 80 to 130° C. or is melt-kneaded with aresin having a reactive functional group at about 100 to 130° C., or thecase where the polycarboxylic acid mixture is dissolved in water, it ispreferred that the content of 1,3,6-hexanetricarboxylic acid in thepolycarboxylic acid mixture is 85% by weight or more, the content ofadipic acid in the polycarboxylic acid mixture is 5% by weight or lessand the content of 3-carboxymethyl-1,5-pentanedicarboxylic acid in thepolycarboxylic acid mixture is 10% by weight or less. It is morepreferred that the content of 1,3,6-hexanetricarboxylic acid is 95% byweight or more, the content of adipic acid is 1% by weight or less andthe content of 3-carboxymethyl-1,5-pentanedicarboxylic acid is 4% byweight or less. It is most preferred that the content of1,3,6-hexanetricarboxylic acid is 98% by weight or more.

When the content of 1,3,6-hexanetricarboxylic acid in the polycarboxylicacid mixture is less than 80% by weight, it is likely that, in variouscases, the polycarboxylic acid mixture cannot exhibit characteristicsthat are inherently possessed by 1,3,6-hexanetricarboxylic acid. Forexample, when the polycarboxylic acid mixture has a content of1,3,6-hexanetricarboxylic acid of less than 80% by weight and has acontent of a polycarboxylic acid (having a relatively low solubility inwater), such as adipic acid, of 10% by weight or more, it is likely thatthe polycarboxylic acid mixture is not uniformly dissolved in an aqueousmedium, such as water.

The content of a polycarboxylic acid (such as 1,3,6-hexanetricarboxylicacid) in the polycarboxylic acid mixture is obtained from the area ofthe peak ascribed to the polycarboxylic acid, wherein the peak ismeasured by liquid chromatography using a refractive index (RI)detector.

The polycarboxylic acid mixture of the present invention has acharacteristic feature with respect to color tone. Specifically, thepolycarboxylic acid mixture of the present invention has a psychometriclightness L-value of 98 or more, a psychometric chroma a-value of from−2.0 to 2.0 and a psychometric chroma b-value of from −2.0 to 3.0.

The psychometric lightness L-value, psychometric chroma a-value andpsychometric chroma b-value of the polycarboxylic acid mixture aremeasured in accordance with JIS Z 8722, wherein the measurement isconducted at 25° C. with respect to a solution obtained by dissolving0.400 g of the polycarboxylic acid mixture in 4.0 ml of distilled water.Specifically, these values are obtained as follows. The spectraltransmission of the above solution is measured by spectrometry using thestandard lights of type C having wave lengths of from 380 to 780 nm toobtain X, Y and Z tristimulus values in the XYZ-space. Using theobtained X, Y and Z values, the L-, a- and b-values are calculated bythe following Hunter's color difference equation prescribed in JIS Z8730:L=10Y^(0.5),a=17.5 (1.02X−Y)/Y ^(0.5) andb=7.0 (Y−0.847Z)/Y ^(0.5)wherein L, a and b represent the psychometric lightness L-value,psychometric chroma a-value and psychometric chroma b-value in theHunter's color difference equation, respectively, and X, Y and Zrepresent the X, Y and Z tristimulus values in the XYZ-space,respectively.

Generally, the psychometric lightness L-value is a yardstick forwhiteness and the upper limit value thereof is 100. The higher thepsychometric lightness L-value of a sample, the higher the whiteness ofthe sample. On the other hand, the lower the psychometric lightnessL-value of the sample, the higher the blackness of the sample. Thepsychometric chroma a-value is a yardstick for greenness and redness.When the psychometric chroma a-value of the sample is 0, the greennessand redness of the sample are each 0. In the case where the sample has apsychometric chroma a-value of less than 0, the lower the psychometricchroma a-value of the sample, the higher the greenness of the sample; onthe other hand, in the case where the sample has a psychometric chromaa-value of higher than 0, the higher the psychometric chroma a-value ofthe sample, the higher the redness of the sample. The psychometricchroma b-value is a yardstick for blueness and yellowness. When thepsychometric chroma b-value of the sample is 0, the blueness andyellowness of the sample are each 0. In the case where the sample has apsychometric chroma b-value of less than 0, the lower the psychometricchroma b-value of the sample, the higher the blueness of the sample; onthe other hand, in the case where the sample has a psychometric chromab-value of higher than 0, the higher the psychometric chroma b-value ofthe sample, the higher the yellowness of the sample.

When the sample has a psychometric lightness L-value of 100, apsychometric chroma a-value of 0 and a psychometric chroma b-value of 0,this means that the sample is colorless. Very close to these values are,respectively, the psychometric lightness L-value, psychometric chromaa-value and psychometric chroma b-value of the polycarboxylic acidmixture of the present invention. This means that the polycarboxylicacid mixture of the present invention has a very excellent color tone(i.e., not discolored, substantially colorless and transparent).

As mentioned above, the polycarboxylic acid mixture of the presentinvention has a psychometric lightness L-value of 98 or more, apsychometric chroma a-value of from −2.0 to 2.0 and a psychometricchroma b-value of from −2.0 to 3.0. The closer to 100 the psychometriclightness L-value of the polycarboxylic acid mixture, the more preferredthe polycarboxylic acid mixture. The psychometric lightness L-value ofthe polycarboxylic acid mixture is preferably 99 or more, morepreferably 99.5 or more. The closer to 0 the psychometric chroma a-valueof the polycarboxylic acid mixture, the more preferred thepolycarboxylic acid mixture. The psychometric chroma a-value of thepolycarboxylic acid mixture is preferably from −1.0 to 1.0, morepreferably from −0.5 to 0.5, most preferably from −0.2 to 0.2. Thecloser to 0 the psychometric chroma b-value of the polycarboxylic acidmixture, the more preferred the polycarboxylic acid mixture. Thepsychometric chroma b-value of the polycarboxylic acid mixture ispreferably from −1.0 to 1.0, more preferably from −0.5 to 0.5. When evenone of the psychometric lightness L-value, psychometric chroma a-valueand psychometric chroma b-value of the polycarboxylic acid mixture fallsoutside the above-mentioned range, a disadvantage is likely to be causedwherein, when the polycarboxylic acid mixture is used, for example, forproducing a paint, the produced paint is discolored to assume a color ofyellow or gray. Further, when even one of the psychometric lightnessL-value, psychometric chroma a-value and psychometric chroma b-value ofthe polycarboxylic acid mixture falls outside the above-mentioned range,the heat stability (i.e., color tone stability under heating) of thepolycarboxylic acid mixture becomes poor and the below-described colordifference (ΔE) of the polycarboxylic acid mixture is far more than 2.Accordingly, when such a polycarboxylic acid mixture is used forproducing a paint which is curable by heating, it is likely that theproduced paint becomes discolored to assume a color of yellow or thelike when the paint is cured by heating.

The polycarboxylic acid mixture of the present invention has a nitrogencontent of 5,000 ppm by weight or less. The nitrogen content of thepolycarboxylic acid mixture is preferably 1,000 ppm by weight or less,more preferably 500 ppm by weight or less. When the nitrogen content ofthe polycarboxylic acid mixture is more than 5,000 ppm, the heatstability of the polycarboxylic acid mixture becomes poor. Specifically,when such a polycarboxylic acid mixture is heated at 80° C. or more, itis likely that the polycarboxylic acid mixture becomes discolored orthat the polycarboxylic acids of the mixture are caused to be modifiedto form nitrogen-containing compounds, such as an amide, an imide and apolymer having an amide bond or an imide bond.

In the present invention, the nitrogen content of the polycarboxylicacid mixture is calculated from the nitrogen concentration of a gaswhich is generated when the polycarboxylic acid mixture is burned at800° C.

Nitrogen contained in the polycarboxylic acid mixture of the presentinvention can be present in the form of various nitrogen-containingcompounds, groups or ions. For example, the nitrogen can be present inthe form of a carboxylate (such as an ammonium salt of a carboxylicacid), an ammonium ion as a component of an inorganic salt, or a group(such as an amide group or an imide group). Examples of inorganic saltsinclude ammonium chloride and ammonium sulfate.

By virtue of the fact that the polycarboxylic acid mixture of thepresent invention has a nitrogen content of 5,000 ppm or less, thepolycarboxylic acid mixture exhibits excellent heat stability (colortone stability under heating). Specifically, the color difference (ΔE)of the polycarboxylic acid mixture represented by the following equationis generally 2 or less, preferably 1 or less:ΔE=[(ΔL)²+(Δa)²+(Δb)²]^(1/2)wherein ΔL represents the difference in the psychometric lightnessL-value of the polycarboxylic acid mixture as between before and afterthe polycarboxylic acid mixture is heated at 80° C. for 18 hours, Δarepresents the difference in the psychometric chroma a-value of thepolycarboxylic acid mixture as between before and after thepolycarboxylic acid mixture is heated at 80° C. for 18 hours, and Δbrepresents the difference in the psychometric chroma b-value of thepolycarboxylic acid mixture as between before and after thepolycarboxylic acid mixture is heated at 80° C. for 18 hours.

The above-mentioned color difference (ΔE) of the polycarboxylic acidmixture is a yardstick for the heat stability (i.e., color tonestability under heating) of the polycarboxylic acid mixture. The closerto 0 the color difference of the polycarboxylic acid mixture, the higherthe heat stability of the polycarboxylic acid mixture.

Further, it is preferred that the polycarboxylic acid mixture hasanother type of color difference of 10 or less, wherein this type ofcolor difference is measured in substantially the same manner as in theabove-mentioned color difference (ΔE), except that the heat treatment ofthe polycarboxylic acid mixture is conducted at 160° C. for 3 hours.

In this connection, the following should be noted. In the methoddescribed in the above-mentioned Agric. Biol. Chem., 45 (1) 57-62, 1981,ammonium chloride, which has high solubility in water, is by-producedduring the hydrolysis of 1,3,6-tricyanohexane. This by-produced ammoniumchloride cannot be satisfactorily removed, so that even a crystal of1,3,6-hexanetricarboxylic acid obtained by recrystallization asdescribed in the Agric. Biol. Chem. above has a nitrogen content aslarge as 8,000 ppm or more. Therefore, the crystal of1,3,6-hexanetricarboxylic acid becomes markedly discolored when thecrystal is heated, for example, at 160° C. for 3 hours or more.

With respect to the raw materials for producing the polycarboxylic acidmixture of the present invention, there is no particular limitation. Forexample, the polycarboxylic acid mixture can be obtained from ahydrolysis reaction mixture which is obtained by hydrolyzing a nitrilemixture comprised mainly of 1,3,6-tricyanohexane, wherein the nitrilemixture is obtained as a by-product in a process for producingadiponitrile from acrylonitrile by electrodimerization or obtained byreacting acrylonitrile with adiponitrile. The above-mentioned nitrilemixture is generally markedly discolored.

Hereinbelow, explanations are given with respect to a method forproducing the polycarboxylic acid mixture of the present invention inhigh yield. The polycarboxylic acid mixture of the present invention canbe produced in high yield by a method which comprises the steps of:

-   -   (1) adjusting the pH value of an aqueous hydrolysis reaction        mixture solution obtained by hydrolyzing, in an aqueous medium,        a nitrile mixture comprised mainly of 1,3,6-tricyanohexane to a        level in the range of from 3 to 13, thereby obtaining an aqueous        solution containing a salt of a polycarboxylic acid mixture,    -   the nitrile mixture being obtained as a by-product in a process        for producing adiponitrile from acrylonitrile by        electrodimerization or obtained by reacting acrylonitrile with        adiponitrile,    -   (2) treating the aqueous solution with a solid adsorbent to        obtain a treated aqueous solution,    -   (3) converting the salt of a polycarboxylic acid mixture in the        treated aqueous solution obtained in step (2) to a        polycarboxylic acid mixture using an ion exchange resin, an        electrodialyzer or an acid, thereby obtaining an aqueous        solution containing a polycarboxylic acid mixture, and    -   (4) recovering the polycarboxylic acid mixture from the aqueous        solution obtained in step (3),    -   wherein when the acid is used in step (3) for converting the        salt to the polycarboxylic acid mixture, the recovered        polycarboxylic acid mixture is subjected to an extraction with        an organic solvent for the polycarboxylic acid mixture to obtain        the polycarboxylic acid mixture as an extract with the organic        solvent, followed by separation of the polycarboxylic acid        mixture in the extract from the organic solvent.

Explanations are given with respect to the above-mentioned step (1). Instep (1), the pH value of an aqueous hydrolysis reaction mixturesolution obtained by hydrolyzing, in an aqueous medium, a nitrilemixture comprised mainly of 1,3,6-tricyanohexane is adjusted to a levelin the range of from 3 to 13, thereby obtaining an aqueous solutioncontaining a salt of a polycarboxylic acid mixture.

The above-mentioned nitrile mixture is obtained as a by-product in aprocess for producing adiponitrile from acrylonitrile byelectrodimerization or obtained by reacting acrylonitrile withadiponitrile. Therefore, the nitrile mixture is generally markedlydiscolored.

Explanations are given with respect to a method for obtaining theabove-mentioned nitrile mixture as a by-product in a process forproducing adiponitrile from acrylonitrile by electrodimerization.

When acrylonitrile is subjected to electrodimerization, a discoloredreaction mixture which is comprised mainly of adiponitrile and containsvarious nitrile compounds (i.e., cyano group-containing compounds) isobtained. The discolored reaction mixture also contains acrylonitrilewhich remains unreacted in the electrodimerization. Examples ofcompounds obtained as by-products in the electrodimerization ofacrylonitrile include propionitrile, α-methylglutaronitrile,hydroxypropionitrile, succinonitrile, a nitrile compound containing 3cyano groups (hereinafter, such a nitrile compound is frequentlyreferred to as “trinitrile compound”) and a nitrile compound containing4 or more cyano groups (hereinafter, such a nitrile compound isfrequently referred to as “polynitrile compound”). Specific examples oftrinitrile compounds include 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane.

It is described in J. Org. Chem., 30 (5) 1351 (1965) that, in theelectrodimerization of acrylonitrile, a trinitrile compound and/or apolynitrile compound produced in an amount which is not negligible.

Generally, as a main example of a trinitrile compound by-produced in theelectrodimerization of acrylonitrile, there can be mentioned1,3,6-tricyanohexane. In the electrodimerization of acrylonitrile,3-cyano-methyl-1,5-dicyanopentane, which is an isomer of1,3,6-tricyanohexane, is also by-produced in a small amount.

A compound (such as acrylonitrile or adiponitrile) having a boilingpoint lower than that of a trinitrile compound, and optionally apolynitrile compound are removed from the above-mentioned discoloredreaction mixture by extraction using a solvent or by distillation underreduced pressure, thereby obtaining the above-mentioned nitrile mixturecomprised mainly of 1,3,6-tricyanohexane. The nitrile mixture maycontain not only a compound having a high boiling point but also acompound having a low boiling point which has not been removed by theabove-mentioned removal operation (extraction or distillation).

The total content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane in the above-mentioned nitrile mixtureis preferably 85% by weight or more, more preferably 90% by weight ormore. When the total content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane is less than 85% by weight, the nitrilemixture inevitably contains the above-mentioned compound having a lowboiling point, a polynitrile compound and a discolored compound which isdifficult to identify and determine, wherein the total content of thesecompounds in the nitrile mixture is 15% by weight or more, therebycausing disadvantages wherein the purity of the finally obtainedpolycarboxylic acid mixture (i.e., the total content of thepolycarboxylic acids in the finally obtained polycarboxylic acidmixture) is lowered and the polycarboxylic acid mixture suffersdiscoloration. Further, for improving the purity of the polycarboxylicacid mixture, various processes for purifying the polycarboxylic acidmixture are required after the polycarboxylic acid mixture is obtained.As a result, it is likely that the yield of the polycarboxylic acidmixture finally obtained and the production efficiency are lowered.

In the present invention, with respect to the content of1,3,6-tricyanohexane in the above-mentioned nitrile mixture, there is noparticular limitation so long as the content of1,3,6-hexanetricarboxylic acid or a salt thereof in a polycarboxylicacid mixture or a salt thereof, which is obtained by hydrolyzing theabove-mentioned nitrile mixture, is 80% by weight or more. The contentof 1,3,6-tricyanohexane in the above-mentioned nitrile mixture isgenerally 80% by weight or more.

The nitrile mixture generally contains 3-cyanomethyl-1,5-dicyanopentanein an amount of from 0.01 to 10% by weight, based on the weight of thenitrile mixture. In the present invention, when it is intended, forexample, to obtain, by crystallization, a high purity1,3,6-hexanetricarboxylic acid in high yield, the content of3-cyanomethyl-1,5-dicyanopentane in the nitrile mixture is preferably 5%by weight or less, more preferably 2% by weight or less.

With respect to the electrodimerization of acrylonitrile, explanationsare given below in more detail.

With respect to the electrolytic cell used for the electrodimerizationof acrylonitrile, there is no particular limitation. For example, therecan be used a so-called diaphragm electrolytic cell which comprises acathode compartment containing a cathode, an anode compartmentcontaining an anode, wherein the cathode compartment and the anodecompartment are partitioned by a cation exchange membrane.Alternatively, there can also be used a single electrolytic cell havingno ion exchange membrane as a diaphragm. With respect to electrolysesusing such electrolytic cells, reference can be made to ExaminedJapanese Patent Application Publication No. Sho 45-24128 and UnexaminedJapanese Patent Application Laid-Open Specification Nos. Sho 59-59888and Sho 59-185788.

When the electrodimerization of acrylonitrile is performed using theabove-mentioned diaphragm electrolytic cell, it is generally possible touse a cathode having a high hydrogen overvoltage. Preferred examples ofsuch cathodes include lead, cadmium and a metal alloy comprised mainlyof these metals. As an anode, it is preferred to use a metal having highcorrosion resistance, such as lead, a lead alloy or platinum. Lead and alead alloy are more preferred. As a diaphragm, a cation exchangemembrane can be used. As an anolyte, an aqueous sulfuric acid solutioncan be used. During the electrodimerization, the catholyte comprisesacrylonitrile, adiponitrile, a trinitrile compound, a compound (otherthan mentioned above) by-produced by the electrodimerization, water anda conductivity supporting salt. The catholyte is present in the form ofeither an emulsion comprising an oil phase and an aqueous phase or auniform solution, wherein the later is realized when acrylonitrile ispresent in an excess amount.

As a preferred example of a conductivity supporting salt, there can bementioned a quaternary ammonium salt represented by the followingformula:[NR¹R²R³R⁴]⁺X⁻wherein each of R¹, R² and R³ independently represents a C₁-C₅ alkylgroup, R⁴ represents a C₁-C₁₆ alkyl group and X⁻ represents an anion ofsulfuric acid, carbonic acid, an alkyl sulfuric acid, phosphoric acid orthe like, or a residual group of an organic acid or a multivalentorganic acid.

The pH value of the catholyte is generally in the range of from 5 to 12.

During the electrodimerization, the temperature of the electrolyticliquid in the electrolytic cell is generally in the range from 40 to 60°C. and the current density is generally in the range of from 5 to 50 Aper 1 dm² of the surface area of the cathode. The cathode and anode aredisposed at a distance of 1 to 20 mm through a diaphragm, and each ofthe catholyte and the anolyte is generally flowed through the diaphragmat a linear velocity of 0.1 to 10.0 m/sec.

When the electrodimerization of acrylonitrile is performed using asingle electrolytic cell which has no ion exchange membrane as adiaphragm, it is preferred to use, as a cathode, lead, cadmium, mercuryor a metal alloy comprising at least one metal selected from the groupconsisting of the above-mentioned metals; and it is preferred to use, asan anode, iron, nickel or an alloy of such metal. The electrolyticliquid is comprised mainly of an alkali metal salt, the above-mentionedquaternary ammonium salt and water. During the electrodimerization, theelectrolytic liquid comprises the above-mentioned compounds (i.e., analkali metal salt, the above-mentioned quaternary ammonium salt andwater), acrylonitrile, adiponitrile, a trinitrile compound and acompound (other than mentioned above) by-produced by theelectrodimerization, and is present in the form of an emulsion or auniform solution. Examples of cations of alkali metal salts includecations of lithium, sodium, potassium and rubidium. Examples of anionsof alkali metal salts include anions of alkali metal salts of inorganicacids (such as phosphoric acid, boric acid, carbonic acid and sulfuricacid) and residual groups of multivalent acids.

With respect to the method for obtaining the above-mentioned nitrilemixture (comprised mainly of 1,3,6-tricyanohexane) from the electrolyticliquid after completion of the electrodimerization of acrylonitrile,there is no particular limitation. Examples of such methods forobtaining the nitrile mixture include a conventional extraction methodand a conventional distillation. These methods can be used individuallyor in combination.

For example, when the electrolytic liquid after completion of theelectrodimerization of acrylonitrile takes a form of an emulsioncomprising an oil phase and an aqueous phase, the nitrile mixture isobtained as follows. Low boiling point compounds, such as acrylonitrileremaining unreacted, propionitrile (which is by-produced by theelectrodimerization), are removed from the emulsion by distillation. Theresultant emulsion is subjected to emulsion destruction to separate theemulsion into an oil phase and an aqueous phase, wherein the aqueousphase contains inorganic compounds and a quaternary ammonium salt andthe oil phase contains water in a small amount, low boiling pointcompounds, and high boiling point compounds (such as adiponitrile and atrinitrile compound).

On the other hand, when the electrolytic liquid after completion of theelectrodimerization of acrylonitrile takes a form of a uniform solution,the nitrile mixture is obtained, for example, as follows. Water and anonaqueous organic solvent, such as methylene chloride, are added to theelectrolytic liquid to thereby extract inorganic salts and a quaternaryammonium salt into the aqueous phase and extract high boiling pointcompounds (such as adiponitrile and a trinitrile compound) into the oilphase.

In either of the above-mentioned two cases, a compound (such asadiponitrile) having a boiling point lower than that of a trinitrilecompound is removed by a conventional distillation or the like, therebyobtaining, as a residue, a high boiling point mixture containing theabove-mentioned nitrile mixture comprised mainly of1,3,6-tricyanohexane. As mentioned above, the high boiling point mixture(as a distillation residue) containing the nitrile mixture contains notonly low boiling point compounds (such as adiponitrile) which has notbeen removed by the operation for removing low boiling point compounds,but also high boiling point compounds, such as a polynitrile compound.

In the high boiling point mixture as a distillation residue, when thetotal content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane is 85% by weight or less, it is alsopossible to perform a distillation at least one more time for removing,from the residue, low boiling point compounds (such as adiponitrile) andoptionally high boiling point compounds, thereby obtaining a nitrilemixture having a total content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane of 85% by weight or more. As preferredexamples of such a distillation, there can be mentioned a moleculardistillation and a thin film distillation which are described inUnexamined Japanese Patent Application Laid-Open Specification No. Sho62-270550. Further, in the present invention, for increasing theabove-mentioned total content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane in the above-mentioned residue to 85%by weight or more, the following operation can be performed. The highboiling point mixture residue is subjected to an extraction with asolvent which can dissolve only 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane, followed by removal of the solventfrom the resultant extract. By this operation, a nitrile mixture havinga total content of 1,3,6-tricyanohexane and3-cyanomethyl-1,5-dicyanopentane of 85% by weight or more can beobtained.

With respect to the method for obtaining the nitrile mixture by reactingacrylonitrile with adiponitrile, explanations are made below.

With respect to the method for obtaining the nitrile mixture by reactingacrylonitrile with adiponitrile, there is no particular limitation, andany conventional method can be employed. For example, a nitrile mixturecomprised mainly of 1,3,6-tricyanohexane can be obtained by a methoddescribed in Examined Japanese Patent Application Publication No. Sho61-3780, in which acrylonitrile is reacted with adiponitrile in thepresence of a basic catalyst, such as a metal alcoholate of isopropylalcohol or t-butyl alcohol.

With respect to each of the nitrile mixtures obtained by theabove-mentioned methods, it is preferred that the content ofadiponitrile in the nitrile mixture is 10% by weight or less and thatthe nitrile mixture is so pure that the total content of1,3,6-tricyanohexane and 3-cyanomethyl-1,5-dicyanopentane in the nitriuemixture is 85% by weight or more.

The aqueous solution obtained in step (1) is an aqueous solution of ahydrolysis reaction mixture obtained by hydrolyzing, in an aqueousmedium, the nitrile mixture obtained by any of the above-mentionedmethods. With respect to the hydrolysis of the nitrile mixture,explanations are made below.

The hydrolysis of the nitrile mixture is performed using an alkali or anacid in an aqueous medium. The term “aqueous medium” means any of waterand an organic solvent which contains water in an amount sufficient forthe hydrolysis of the nitrile mixture and does not adversely affect thehydrolysis reaction. As an aqueous medium, water is preferred.

With respect to the alkali used for the hydrolysis of the nitrilemixture, there is no particular limitation so long as it is a compoundwhich exhibits alkaline property in an aqueous solution thereof.Examples of alkalis include alkali metal compounds, alkaline earth metalcompounds and nitrogen compounds.

Examples of alkali metal compounds include alkali metal hydroxides, suchas lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidiumhydroxide and cesium hydroxide; alkali metal carbonates, such as lithiumcarbonate, sodium carbonate, potassium carbonate, rubidium carbonate andcesium carbonate; alkali metal hydrogencarbonates, such as lithiumhydrogencarbonate, sodium hydrogencarbonate, potassiumhydrogencarbonate, rubidium hydrogencarbonate and cesiumhydrogencarbonate; and alkali metal alcoxides, such as potassiumbutoxide, potassium ethoxide, potassium methoxide, sodium butoxide,sodium ethoxide, sodium methoxide, lithium butoxide, lithium ethoxideand lithium methoxide.

Examples of alkaline earth metals compounds include alkaline earth metalhydroxides, such as beryllium hydroxide, magnesium hydroxide, calciumhydroxide, strontium hydroxide, barium hydroxide and radium hydroxide;alkaline earth metal carbonates, such as beryllium carbonate, magnesiumcarbonate, calcium carbonate, strontium carbonate, barium carbonate andradium carbonate; and alkaline earth metal hydrogencarbonates, such asberyllium hydrogencarbonate, magnesium hydrogencarbonate, calciumhydrogencarbonate, strontium hydrogencarbonate, barium hydrogencarbonateand radium hydrogencarbonate.

Examples of nitrogen compounds include ammonia and various amines.

Among the above-exemplified alkalis, sodium hydroxide and potassiumhydroxide are preferred.

The above-mentioned alkalis can be used individually or in combination.

With respect to the acid used for the hydrolysis of the nitrile mixture,there is no particular limitation so long as it exhibits acid propertyin an aqueous solution thereof. Examples of acids include inorganicacids, such as hydrochloric acid, sulfuric acid and nitric acid; andorganic acids, such as carboxylic acids and sulfonic acids.

When the hydrolysis of the nitrile mixture is performed using an aqueousalkali solution, there is no particular limitation with respect to thealkali concentration of the aqueous alkali solution. However, the alkaliconcentration of the aqueous alkali solution is generally in the rangeof from 1.0 to 50% by weight. When the hydrolysis is performed using,for example, sodium hydroxide as an alkali under atmospheric pressure,the alkali concentration of the aqueous alkali solution is generally inthe range of from 2.0 to 40.0% by weight, preferably from 10.0 to 30.0%by weight. When the alkali concentration of the aqueous alkali solutionis less than 1.0% by weight, the rate of the hydrolysis is lowered. Onthe other hand, when the alkali concentration of the aqueous alkalisolution is more than 50.0% by weight, the solubility of1,3,6-tricyanohexane in the aqueous phase, which is the site for thereaction of 1,3,6-tricyanohexane, is extremely lowered and, hence, therate of the hydrolysis is likely to be markedly lowered.

The equivalent ratio of the alkali to 1,3,6-tricyanohexane istheoretically 1.00 or more in terms of the equivalent ratio of thehydroxyl ion in the alkali to the cyano group in 1,3,6-tricyanohexane.However, for obtaining a satisfactory rate of the hydrolysis, there isgenerally used an equivalent ratio in the range of from 1.01 to 3.00,preferably from 1.05 to 2.00. When the equivalent ratio is more than3.00, the alkali remains in an excess amount in the hydrolysis reactionsystem, so that a large burden is likely to be imposed for removing thealkali present in an excess amount.

In the hydrolysis by using the above-mentioned alkali, a mixture ofalkali salts of polycarboxylic acids is produced.

When the hydrolysis of the nitrile mixture is performed using an acid,the acid concentration of the aqueous acid solution varies depending onthe type of the acid. However, the acid concentration is generally inthe range of from 1 to 98% by weight. For example, when hydrochloricacid is used as the acid, the acid concentration of the hydrochloricacid is generally in the range of from 2 to 37% by weight, preferablyfrom 20 to 37% by weight. When the acid concentration of thehydrochloric acid is less than 2.0% by weight, the rate of thehydrolysis is likely to be lowered. On the other hand, when the acidconcentration of the hydrochloric acid is more than 37% by weight, itbecomes difficult to obtain such a hydrochloric acid having a high acidconcentration in an amount which is sufficient for a commercial scaleproduction of the polycarboxylic acid mixture. When sulfuric acid isused as the acid, the acid concentration of the aqueous sulfuric acidsolution is generally in the range of from 2 to 85% by weight,preferably from 20 to 60% by weight. When the acid concentration of theaqueous sulfuric acid solution is less than 2% by weight, the rate ofthe hydrolysis is likely to be lowered. On the other hand, when the acidconcentration of the aqueous sulfuric acid solution is more than 85% byweight, the following disadvantage is likely to occur. When the aqueoussulfuric acid solution has a sulfuric acid concentration as high as morethan 85% by weight, the amount of water necessary for the intendedhydrolysis inevitably becomes small. Therefore, for achieving an amountof water which is sufficient for the hydrolysis, too large an amount ofsulfuric acid is needed.

The equivalent ratio of the acid to 1,3,6-tricyanohexane is generally inthe range of from 1.01 to 5.0, preferably from 1.05 to 3.0, in terms ofthe ratio of the acid to the cyano group in 1,3,6-tricyanohexane. Whenthe equivalent ratio is less than 1.01, the rate of the hydrolysis isunpractically low. On the other hand, when the equivalent ratio is morethan 5.0, an excess amount of the acid remains unreacted in the reactionsystem and, hence, a step for removing the acid becomes necessary.

In either of the case where the hydrolysis is performed using an alkaliand the case where the hydrolysis is performed using an acid, thetemperature for the hydrolysis is generally in the range of from 50 to250° C., preferably from 80 to 140° C. When the hydrolysis reactiontemperature is lower than 50° C., the rate of the hydrolysis is lowered.When the hydrolysis reaction temperature is higher than 250° C., adisadvantage is likely to occur wherein side reactions, such asdecomposition, occur. The hydrolysis reaction time is generally in therange of from 1 to 200 hours. With respect to the hydrolysis reactionpressure, there is no particular limitation and the hydrolysis may beperformed under superatmospheric pressure, atmospheric pressure orreduced pressure.

With respect to the atmosphere for the hydrolysis of the nitrilemixture, there is no particular limitation so long as no side reactionoccur. For example, the hydrolysis may be performed in an atmosphere ofan inert gas (such as nitrogen) or in air. When the hydrolysis isperformed under atmospheric pressure, the reactor used for thehydrolysis may have an apparatus for cooling water which has beenvaporized to recycle the resultant cooled water to the hydrolysisreaction system or have an apparatus for causing the liquid phase tobubble using a gas, such as nitrogen or air, thereby purging ammoniadissolved in the liquid phase from the hydrolysis reaction system. Whenthe hydrolysis is performed under superatmospheric pressure, it ispreferred that the reactor is equipped with an apparatus for purgingby-produced ammonia from the hydrolysis reaction system.

In the present invention, the hydrolysis of the nitrile mixture may beeither a one-step reaction or a two-step reaction. For example, when anamide is present as an intermediate in the hydrolysis reaction systemafter the hydrolysis using an alkali, an additional hydrolysis may beperformed using another alkali or an acid.

The aqueous solution of a hydrolysis reaction mixture obtained by thehydrolysis of the nitrile mixture contains, as an impurity, nitrogen inthe form of a compound thereof.. For example, when the hydrolysis isperformed using an acid, the aqueous hydrolysis reaction mixturesolution contains an ammonium salt of the acid in an amount which isequivalent to that of the cyano group in the 1,3,6-tricyanohexane whichis present before the hydrolysis is performed. On the other hand, whenthe hydrolysis of the nitrile mixture is performed using an alkali,ammonia is generated and a part of the ammonia is dissolved in theaqueous hydrolysis reaction mixture solution. If desired, beforeadjusting, in step (1), the pH value of the aqueous hydrolysis reactionmixture solution to a level in the range of from 3 to 13, such anitrogen compound as an impurity may be separated from the aqueoushydrolysis reaction mixture solution.

With respect to a method for separating, before step (1), a nitrogencompound as an impurity from the aqueous hydrolysis reaction mixturesolution, explanations are made below, taking as an example the casewhere the hydrolysis is performed using an acid. When the hydrolysis isperformed using an acid, a polycarboxylic acid mixture is produced andan ammonium salt is also generated as an impurity. As an example of amethod for separating the ammonium salt from the polycarboxylic acidmixture, there can be mentioned a method in which the solvent is removedfrom the hydrolysis reaction mixture, the polycarboxylic acid mixture isdissolved into another solvent which can dissolve the polycarboxylicacid mixture, and the polycarboxylic acid mixture is separated from theammonium salt. In this case, the amount of the ammonium salt containedin the aqueous hydrolysis reaction mixture solution is preferably 0.01%by weight or less, more preferably 0.001% by weight or less, based onthe weight of the polycarboxylic acid mixture in the aqueous hydrolysisreaction mixture solution.

In step (1) of the method of the present invention, the pH value of theaqueous hydrolysis reaction mixture solution (which mixture comprisesthe polycarboxylic acid mixture or a salt thereof) obtained by theabove-mentioned hydrolysis is adjusted to a level in the range of from 3to 13, thereby obtaining an aqueous solution containing a salt of thepolycarboxylic acid mixture. The salt of the polycarboxylic acid mixturemay contain a small amount of the polycarboxylic acid mixture. In step(2) of the method of the present invention, the aqueous solutionobtained in step (1) is treated with a solid adsorbent for the purposeof effecting decoloration of the aqueous solution because theabove-mentioned aqueous hydrolysis reaction mixture solution isgenerally markedly discolored.

The term “solid adsorbent” means a substance exhibiting no fluidity butmaintaining the form of a solid at 25° C. and also at the temperature atwhich an adsorption treatment is performed, wherein the substance has aninterface which causes a positive adsorption. Specific examples of solidadsorbents include oxides and hydroxides of metals, such as aluminum,iron, titanium, silicon and tin; activated carbon; bentonite; activatedclay; diatomaceous earth; zeolites; hydrotalcites; cation exchangeresins; and anion exchange resins. Examples of the above-mentionedoxides and hydroxides of aluminum, iron, titanium, silicon and tininclude activated alumina, silica gel and titanium oxide. In the presentinvention, activated carbon, activated alumina and silica gel arepreferred, because they exhibit high decoloration efficiency.

These solid adsorbents may be used individually or in combination. Whentwo or more types of solid adsorbents are used in combination, thesesolid adsorbents may be used either simultaneously or individually. Withrespect to the morphology and size of the solid adsorbent, there is noparticular limitation.

With respect to the amount of the solid adsorbent, there is noparticular limitation so long as the polycarboxylic acid mixture of thepresent invention can be obtained. However, the amount of the solidadsorbent is generally from 0.01 to 500 parts by weight, preferably from0.1 to 100 parts by weight, more preferably from 0.1 to 50 parts byweight, relative to 100 parts by weight of the above-mentioned salt ofthe polycarboxylic acid mixture. When the amount of the solid adsorbentis less than 0.01 part by weight, the decoloration effect is notsatisfactory. On the other hand, when the amount of the solid adsorbentis more than 500 parts by weight, the decoloration effect issatisfactory, but the yield of the polycarboxylic acid mixture is likelyto be lowered.

In step (2), the pH value of the above-mentioned aqueous hydrolysisreaction mixture solution to be treated with a solid adsorbent is veryimportant. When the hydrolysis is performed using an alkali, the pHvalue of the aqueous hydrolysis reaction mixture solution is generallymore than 13. On the other hand, when the hydrolysis is performed usingan acid, the pH value of the aqueous hydrolysis reaction mixturesolution is generally less than 3. When an aqueous hydrolysis reactionmixture solution having such a pH value is directly treated with a solidadsorbent, it is likely that the decoloration efficiency is lowered andthe polycarboxylic acid mixture of the present invention havingexcellent color tone cannot be obtained.

With respect to the method for adjusting the pH value of the aqueoushydrolysis reaction mixture solution, there is no particular limitation.Examples of methods for adjusting the pH value of the aqueous hydrolysisreaction mixture solution include a method in which an inorganic acid(such as hydrochloric acid, sulfuric acid or nitric acid), an organicacid (such as acetic acid or 1,3,6-hexatricarboxylic acid) or an alkalimetal hydroxide (such as sodium hydroxide or potassium hydroxide) isadded to the aqueous hydrolysis reaction mixture solution; and a methodin which the aqueous hydrolysis reaction mixture solution is contactedwith an ion exchange resin.

With respect to the period of time for the treatment with a solidadsorbent, there is no particular limitation so long as thepolycarboxylic acid mixture of the present invention can be obtained.The treatment time is generally from 1 minute to 10 hours, preferablyfrom 5 minutes to 5 hours, more preferably from 10 minutes to 2 hours.When the treatment time is less than 1 minute, the decoloration effectis likely to be unsatisfactory. On the other hand, when the treatmenttime is more than 10 hours, the decoloration effect is satisfactory, butthe production efficiency is lowered. With respect to the method foradsorption by a solid adsorbent, there is no particular limitation.Examples of such adsorption methods include a method in which a solidadsorbent is added to the aqueous solution containing the polycarboxylicacid mixture, followed by stirring; and a method in which theabove-mentioned aqueous solution is introduced into a column which ispacked with a solid adsorbent.

With respect to the concentration of the polycarboxylic acid mixture ora salt thereof in the above-mentioned aqueous solution to be treatedwith a solid adsorbent, there is no particular limitation. However, theconcentration is generally in the range of from 0.02 to 2.0 mol/l,preferably from 0.1 to 1.5 mol/l. When the concentration is less than0.02 mol/l, the amount of the aqueous solution to be treated is largeand the recovery of the polycarboxylic acid mixture or a salt thereofbecomes difficult, leading to a lowering of the yield of thepolycarboxylic acid mixture or a salt thereof. On the other hand, whenthe concentration is more than 2.0 mol/l, a mixture comprising thepolycarboxylic acid mixture and a salt thereof is deposited and theadsorption of a discolored substance by the solid adsorbent becomesunsatisfactory.

With respect to the temperature for the treatment with a solidadsorbent, there is no particular limitation so long as theabove-mentioned salt of the polycarboxylic acid mixture is notsolidified or decomposed. For example, when activated carbon is used asa solid adsorbent and water is used as a solvent, the treatmenttemperature is generally in the range of from 5 to 100° C.

With respect to the method for separating and removing the solidadsorbent which remains in the aqueous solution after the adsorptiontreatment, there is no particular limitation, and the separation andremoval can be performed by a conventional method (such as filtration)which is generally employed in the art.

In step (3), the salt of a polycarboxylic acid mixture in the treatedaqueous solution obtained in step (2) is converted to a polycarboxylicacid mixture using an ion exchange resin, an electrodialyzer or an acid,thereby obtaining an aqueous solution containing a polycarboxylic acidmixture. In step (4), the polycarboxylic acid mixture is recovered fromthe aqueous solution obtained in step (3). In step (3), when theconversion of the salt of a polycarboxylic acid mixture to apolycarboxylic acid mixture is performed using an ion exchange resin oran electrodialyzer, a nitrogen compound which has remained as animpurity in the treated aqueous solution obtained in step (2) is removedsimultaneously with the conversion of the salt of a polycarboxylic acidmixture to a polycarboxylic acid mixture.

In step (3), the pH value of the treated aqueous solution is adjusted toa level lower than 3 to thereby convert the salt of a polycarboxylicacid mixture in the aqueous solution to a polycarboxylic acid mixture.When the pH value of the aqueous solution is 3 or more, a polycarboxylicacid in the aqueous solution contains a salt or partial salt thereofwith a basic compound and the content of a free carboxylic acid in theaqueous solution is low.

The pH value of the aqueous solution is adjusted to a level which islower than 3, preferably lower than 2.5. The adjusted pH value variesdepending on the total concentration of the polycarboxylic acids in theaqueous solution; however, it is more preferred that the pH value of theaqueous solution is adjusted to a level lower than 2.3.

The acid used in step (3) for adjusting the pH value of the aqueoussolution may be either an inorganic acid or an organic acid. Examples ofinorganic acids include sulfuric acid, hydrochloric acid and nitricacid. Examples of organic acids include carboxylic acids, such as formicacid and acetic acid; and sulfonic acids, such as methanesulfonic acid.When the adjustment of the pH value of the aqueous solution to levellower than 3 is performed using any one of the above-mentioned inorganicacids and organic acids, it is necessary to separate free polycarboxylicacids (which is formed by the addition of the acid for adjusting the pHvalue of the aqueous solution) from salts derived from the acid foradjusting the pH value of the aqueous solution. As the acid foradjusting the pH value of the aqueous solution, an inorganic acid ispreferred, because an inorganic acid generally has a high acidity and,hence, the amounts of free polycarboxylic acids which are formed by theaddition of the acid are likely to be large.

When the conversion of the salt of a polycarboxylic acid mixture to apolycarboxylic acid mixture is performed using an inorganic acid, forseparating a by-produced inorganic salt from the aqueous solutioncontaining the polycarboxylic acid mixture, the aqueous solution issubjected to an extraction with a solvent for the polycarboxylic acidmixture to obtain the polycarboxylic acid mixture as an extract with theorganic solvent. By the extraction, a nitrogen compound which hasremained in the aqueous solution is removed together with theabove-mentioned inorganic salt. Examples of extraction solvents includetert-butyl methyl ether and tetrahydrofuran. Other solvents whichexhibit the same performance as those of the above-mentioned solventscan also be used. With respect to the aqueous solution containing thepolycarboxylic acid mixture, it is also possible to perform thefollowing operation. The aqueous solution is subjected to distillationunder heating and reduced pressure to obtain a completely dry residue.The residue is subjected to an extraction with an organic solvent forthe polycarboxylic acid mixture to obtain the polycarboxylic acidmixture as an extract with the organic solvent. Examples of organicsolvents used for the extraction include ethers, such astetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether andtert-butyl methyl ether; ketones, such as acetone, methyl ethyl ketoneand methyl isobutyl ketone; acetates, such as methyl acetate and ethylacetate; carbonates, such as dimethyl carbonate and diethyl carbonate;and nitrites, such as acetonitrile and propionitrile. These organicsolvents can be used individually or in combination.

With respect to the method for extraction, there is no particularlimitation, and a conventional method, such as an extraction by stirringor an extraction using a Soxhlet's extractor, can be used.

The solvent can be removed from the extract to recover thepolycarboxylic acid mixture, wherein, if desired, prior to the removalof the solvent from the extract, the extract is dried using a dryingagent, such as magnesium sulfate anhydride.

In step (3), when the conversion of the salt of a polycarboxylic acidmixture to a polycarboxylic acid mixture is performed using a cationexchange resin, the ion exchange can be preferably performed in abatchwise manner or a continuous manner. However, the manner in whichthe ion exchange is performed is not limited to the above-mentionedmanners. Examples of cation exchange resins include a resin(manufactured and sold by Mitsubishi Chemical Corporation, Japan) whichis produced by introducing a sulfonic acid group to an aromatic ring ofa styrene/divinylbenzene copolymer. This resin is, prior to use, treatedwith an acid to regenerate the resin. Representative examples of cationexchange resins of this type include strongly acidic cation exchangeresins, such as Diaion SK102, SK104, SK106, SK1B, SK110, SK112, SK116and SK1BN (trade names; manufactured and sold by Mitsubishi ChemicalCorporation, Japan). It is also possible to use sulfonic acid typestrongly acidic cation exchange resins, such as Amberlyst 15WET, 16WET,31WET and 35WET (trade names; manufactured and sold by Rohm and HaasCo., U.S.A.). With respect to the type of the cation exchange resin,there is no particular limitation.

With respect to the case where an electrodialyzer is used in step (3)(that is, step (3) is performed by electrodialysis), explanations aregiven below. The electrodialysis is performed as follows. The aqueoussolution of the salt of a polycarboxylic acid mixture is caused to beflowed in or circulate through compartments in the electrodialyzer,wherein the compartments are partitioned by ion exchange membranes. Avoltage is applied to the electrodialyzer to remove a basic componentfrom the aqueous solution through the ion exchange membranes, therebylowering the pH value of the aqueous solution to a level lower than 3 tothereby convert the salt of a polycarboxylic acid mixture to apolycarboxylic acid mixture.

In the present invention, the electrodialysis is preferably performedusing an electrodialyzer having conventional ion exchange membranes.Examples of electrodialyzers having conventional ion exchange membranesinclude an electrodialyzer which has cation exchange membranes but hasno anion exchange membranes; an electrodialyzer having cation exchangemembranes and anion exchange membranes; an electrodialyzer having cationexchange membranes and bipolar membranes (amphoteric membranes), anelectrodialyzer having anion exchange membranes and bipolar membranes;and an electrodialyzer having cation exchange membranes, anion exchangemembranes and bipolar membranes.

As a preferred example of an electrodialysis using an electrodialyzerwhich has cation exchange membranes but has no anion exchange membranes,there can be mentioned a method disclosed in Unexamined Japanese PatentApplication Laid-Open Specification No. Sho 50-111010. As a preferredexample of an electrodialysis using an electrodialyzer having cationexchange membranes and anion exchange membranes, there can be mentioneda method disclosed in Unexamined Japanese Patent Application Laid-OpenSpecification No. Hei 2-115025. As a preferred example of anelectrodialysis using an electrodialyzer having cation exchangemembranes and bipolar membranes, there can be mentioned a methoddisclosed in Patent Application Prior-to-Examination Publication (Kohyo)No. Hei 7-507598. As a preferred example of an electrodialysis using anelectrodialyzer having anion exchange membranes and bipolar membranes,there can be mentioned a method disclosed in Unexamined Japanese PatentApplication Laid-Open Specification No. Hei 8-281077.

With respect to the material for an ion exchange membrane used in theelectrodialysis, there is no particular limitation so long as thematerial is generally used in the art. For example, fluorinated olefinresins, styrene/divinylbenzene copolymer resins, olefin resins andchlorinated olefin resins can be used as a material for an ion exchangemembrane.

In the case of a material for a cation exchange membrane, it ispreferred to use a material obtained by adding, to any one of thematerials exemplified above as the materials which are generally used inthe art, at least one group having a negative charge, such as a sulfonicacid group or a carboxyl group. On the other hand, in the case of amaterial for an anion exchange membrane, it is preferred to use amaterial obtained by adding, to any one of the materials exemplifiedabove as the materials which are generally used in the art, at least onefunctional group, such as a quaternary ammonium group, a primary aminogroup, a secondary amino group or a tertiary amino group.

Further, as a bipolar membrane, there can be used a bipolar membraneproduced by a conventional method. As an example of a conventionalmethod for producing a bipolar membrane, there can be mentioned a methodin which a cation exchange membrane and an anion exchange membrane arelaminated through a curable adhesive or a binder (such as a pastecontaining a thermoplastic resin). Examples of methods using a curableadhesive include a method in which a polyethyleneimine/epichlorhydrinmixture is used as a curable adhesive (see Examined Japanese PatentApplication Publication No. Sho 32-3962); and a method using a curableadhesive having an ion-exchange group, wherein the adhesive, uponcuring, takes a cured structure which is ion exchangeable and hasconductivity (see Examined Japanese Patent Application Publication No.Sho 34-3961). Further examples of bipolar membranes include a membraneproduced by a method in which vinyl pyridine and an epoxy compound areapplied onto one surface of a cation exchange membrane, followed bycuring using radiation (see Examined Japanese Patent ApplicationPublication No. Sho 38-16633); a membrane produced by a method in whicha sulfonic acid type polymer electrolyte and an allylamine are attachedto the surface of an anion exchange membrane, followed by ionizationradiation to effect closslinking (see Examined Japanese PatentApplication Publication No. Sho 51-4113); a membrane produced by amethod in which onto a surface of a first ion exchange membrane isattached a resin mixture obtained by dispersing a second ion exchangeresin (having a charge which is opposite to that of the first ionexchange membrane) in a base polymer (see Examined Japanese PatentApplication Publication No. Sho 53-37190); a membrane produced by amethod in which a sheet produced by impregnation-copolymerization of apolyethylene film with styrene or divinyl benzene is inserted in a metalframe (such as a metal frame made of stainless steel), a surface of thesheet (which surface is not contacted with the metal frame) issulfonated, the sheet is released from the metal frame, the othersurface of the film (which is not sulfonated) is chloromethylated and,then, aminated (see U.S. Pat. No. 3,562,139); and a membrane produced bya method in which a specific type of a metal ion is applied onto both asurface of an anion exchange membrane and a surface of a cation exchangemembrane, the anion exchange membrane and the cation exchange membraneare put upon another so that the metal ion-applied surfaces of thecation exchange membrane and anion exchange membrane face each other,followed by pressing (see Electro Chemica Acta Vol. 31, pages 1175 to1176 (1986)).

As an electrode used in the electrodialysis, there can be mentioned aconventional electrode. Preferred examples of anodes include platinum, atitanium/platinum alloy, carbon, nickel, a ruthenium/titanium alloy andan illidium/titanium alloy. Preferred examples of cathodes include iron,nickel, platinum, a titanium/platinum alloy, carbon and stainless steel.The above-mentioned electrodes may have a conventional morphology, suchas a bar, a plate, a mesh or a lattice shape.

In the present invention, when only a cation exchange membrane is usedas an ion exchange membrane, the electrodialysis is performed asfollows. A space between the anode and cathode is partitioned by cationexchange membranes so that a plurality of compartments are formed. Theaqueous solution comprising a salt of a carboxylic acid mixture, and anaqueous solution of a mineral acid (such as sulfuric acid orhydrochloric acid) are circulated through the compartments so thatcompartments containing the aqueous solution comprising a salt of acarboxylic acid mixture are arranged alternately with compartmentscontaining the aqueous solution of a mineral acid. Electrodialysis isperformed so that an aqueous solution comprising a polycarboxylic acidmixture having a pH value of less than 3 and an aqueous mineral acidsalt solution are, respectively, produced in alternately averagedcompartments therefor.

When a bipolar membrane is used in combination with a cation exchangemembrane, it is possible to employ a method in which a mineral acid isintroduced to alternate compartments as mentioned above so as to reactthe mineral acid with a free alkali which is obtained from the aqueoussolution of a salt of a polycarboxylic acid mixture, thereby convertingthe salt of a polycarboxylic acid mixture to a polycarboxylic acidmixture, as in the case where only a cation exchange membrane is used asan ion exchange membrane. Alternatively, it is also possible to employ amethod in which an alkali (which is obtained from the aqueous solutioncomprising a salt of a polycarboxylic acid mixture) is recovered as anaqueous solution of the alkali without using a mineral acid. This methodis especially preferred because the recovered aqueous alkali solutioncan be reused in the hydrolysis of the nitrile mixture.

With respect to the ion exchange membrane used in the electrodialysis,the cut-off molecular weight thereof is generally 2,000 or less,preferably 1,000 or less, more preferably 300 or less. With respect tothe term “cut-off molecular weight”, an explanation is given below,taken as an example the case where the ion exchange membrane has acut-off molecular weight of 300. A 1% by weight aqueous solution ofpolyethylene glycol having a specific molecular weight is contacted withone surface of the ion exchange membrane, whereas distilled water iscontacted with the other surface of the ion exchange membrane. Each ofthe aqueous solution and the distilled water is individually stirred at25° C. under atmospheric pressure for 1 hour. The polyethylene glycol isdiffused into the distilled water. The amount of polyethylene glycoldiffused into the distilled water is measured. If the amount of thepolyethylene glycol diffused is 5% by weight, based on the weight of thepolyethylene glycol in the aqueous solution of polyethylene glycolhaving a molecular weight of 300, the ion exchange membrane is definedto have a cut-off molecular weight of 300. When the cut-off molecularweight of the ion exchange membrane is more than 2,000, a freepolycarboxylic acid which is formed by the electrodialysis is likely toleak through the ion exchange membrane, leading to a lowering of theyield of the polycarboxylic acid mixture.

In view of not only the range of the temperature at which neither a saltor partial salt of a polycarboxylic acid mixture nor a freepolycarboxylic acid does deposit, but also the heat resistance of theion exchange membrane, the electrodialysis temperature is generally inthe range of from 5 to 80° C., preferably from 10 to 60° C.

In the electrodialysis, the current density is generally in the range offrom 0.1 to 100 A/cm², preferably from 0.2 to 50 A/cm².

In the present invention, the mutually adjacent ion exchange membranesmay be disposed at a distance which is generally prescribed in the art.The distance between the mutually adjacent ion exchange membranes isgenerally in the range of from 0.01 to 10 mm, preferably from 0.05 to1.50 mm.

In the present invention, the electrodialysis may be performed either ina batchwise manner or in a continuous manner.

By the above-mentioned method, the polycarboxylic acid mixture comprisedmainly of 1,3,6-hexatricarboxylic acid can be obtained. If it isintended to increase the content of 1,3,6-hexatricarboxylic acid in thepolycarboxylic acid mixture, and also intended to enhance the heatstability and color tone of the polycarboxylic acid mixture, it ispreferred to purify the polycarboxylic acid mixture by a crystallizationmethod in which the polycarboxylic acid mixture is dissolved in asolvent selected from the group consisting of water, an organic solventand a mixture thereof, and the resultant solution is cooled orconcentrated to deposit a crystal.

Examples of organic solvents used for the crystallization method includeethers, such as tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethylether and tert-butyl methyl ether; ketones, such as acetone, methylethyl ketone and methyl isobutyl ketone; acetates, such as methylacetate and ethyl acetate; carbonates, such as dimethyl carbonate anddiethyl carbonate; nitriles, such as acetonitrile and propionitrile;hydrocarbons, such as toluene, xylene, n-hexane and cyclohexane;halogenated hydrocarbons, such as methylene chloride, chloroform andchlorobenzene; and carboxylic acids, such as acetic acid and propionicacid. These solvents can be used individually or in combination.

As examples of methods for crystallization, there can be mentioned amethod in which the polycarboxylic acid mixture is uniformly dissolvedin a solvent at a temperature which is equal to or lower than theboiling point of the solvent, followed by cooling or concentration tothereby deposit a crystal; and a method in which the polycarboxylic acidmixture is dissolved in a first solvent (such as acetone) in which thepolycarboxylic acid mixture has high solubility, the resultant solutionis then introduced into a second solvent in which the polycarboxylicacid mixture has low solubility, to thereby deposit a crystal of thepolycarboxylic acid at room temperature, wherein the deposition can befacilitated by cooling or concentration.

The thus obtained crystal can be recovered by filtration. With respectto the method for filtration, there is no particular limitation. It ispreferred to employ any of filtration under superatmospheric pressure,filtration under reduced pressure, centrifugal separation, andfiltration using pressing.

The polycarboxylic acid mixture having been purified by theabove-mentioned crystallization is advantageous in that, when thecarboxylic acid mixture is used in the field of paints in which thecarboxylic acid mixture is required to be cured by heating,discoloration of the polycarboxylic acid mixture during the curingthereof can be effectively suppressed.

The above-mentioned crystallization can be applied to the aqueoussolution obtained in step (3).

The polycarboxylic acid mixture of the present invention can beadvantageously used as a curing agent for a compound having two or moreepoxy groups in a molecule thereof. That is, in another aspect of thepresent invention, there is provided a curable composition comprising:(a) compound having two or more epoxy groups in a molecule thereof, and(b) a curing agent comprising the polycarboxylic acid mixture of thepresent invention.

The curable composition of the present invention has a high curing rateand is advantageous not only in that a cured composition obtained bycuring the curable composition has excellent mechanical properties, butalso in that discoloration of the curable composition during the curingthereof can be effectively suppressed.

In the present invention, the curing agent (b) means a substance whichcan be reacted with an epoxy group to form a crosslinked structure. Forexample, the polycarboxylic acid mixture of the present invention assuch can be used as the curing agent (b). Alternatively, the curingagent (b) may be a mixture of the polycarboxylic acid mixture of thepresent invention and another curing agent. Examples of curing agentsother than the polycarboxylic acid mixture of the present inventioninclude carboxylic acid compounds, acid anhydrides and amines.

Examples of carboxylic acid compounds include aliphatic, aromatic andalicyclic compounds having two or more carboxyl groups in a moleculethereof. Specific examples of such carboxylic compounds include subericacid, sebacic acid, azelaic acid, decanedicarboxylic acid,cyclohexanedicarboxylic acid, isophthalic acid, trimellitic acid,tetrahydrophthalic acid, hexahydrophthalic acid,1,2,4-butanetricarboxylic acid, a copolymer comprising acrylic acid ormethacrylic acid, a polyester resin having a carboxyl group in aterminal thereof, and a polyamide resin having a carboxyl group in aterminal thereof.

Examples of acid anhydrides include maleic anhydride, phthalicanhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride andtrimellitic anhydride.

As an amine, there can be used a compound having at least one aminogroup in a molecule thereof. Examples of amines include ethylenediamine,diethylenetetramine, triethylenetetramine and hexamethylenediamine.

With respect to the content of the polycarboxylic acid mixture of thepresent invention in the curing agent (b), there is no particularlimitation. However, when it is intended to improve the crosslinkingdensity of a cured composition obtained by curing the curablecomposition, the content of the polycarboxylic acid mixture in thecuring agent (b) is generally 10% by weight or more, preferably 50% byweight or more.

When it is intended to use the polycarboxylic acid mixture of thepresent invention for the curable composition, the content of1,3,6-hexanetricarboxylic acid in the polycarboxylic acid mixture ispreferably 90% by weight or more, more preferably 95% by weight or more,most preferably 98% by weight or more.

With respect to the epoxy compound (a) (i.e., compound having two ormore epoxy groups in a molecule thereof) used in the curable compositionof the present invention, explanations are given.

The term “epoxy group” means a 3-membered ring structure having acarbon-carbon-oxygen linkage. The carbon-carbon linkage in thecarbon-carbon-oxygen linkage may be either a part of a straight chain orbranched hydrocarbon or a part of a cyclic hydrocarbon having a 5- or6-membered ring, wherein each of the straight chain or branchedhydrocarbon and the cyclic hydrocarbon may have bonded thereto a halogenatom (such as a fluorine atom, a chlorine atom or a bromine atom) or afunctional group (such as a hydroxyl group). Further, a carbon atomforming the epoxy group may have bonded thereto an alkyl group (such asa methyl group) or a halogen atom.

Examples of epoxy groups include a glycidyl group and an alicyclic epoxygroup. Among these epoxy groups, a glycidyl group is preferred.

With respect to the structure of the epoxy compound (a), there is noparticular limitation so long as the epoxy compound (a) has two or moreepoxy groups in a molecule thereof. The epoxy compound (a) may be eithera low molecular weight compound having a molecular weight of less than1,000 or a high molecular weight compound having a molecular weight of1,000 or more. Further, the epoxy compound (a) may be a polymer.

Examples of epoxy compounds (a) include a compound having an epoxy group(such as a glycidyl group) bonded to an ether linkage or an esterlinkage; a compound having an epoxy group bonded to a nitrogen atom; anda polymer containing an epoxy group. Depending on the use and desiredproperties of the curable composition of the present invention, theabove-mentioned epoxy compounds can be used individually or incombination.

Examples of epoxy compounds having a glycidyl group bonded to an etherlinkage include ethylene glycol diglycidyl ether, propanediol diglycidylether, butanediol diglycidyl ether, hydrogenated bisphenol A diglycidylether, diglycidyl ether obtained from hydroquinone, diglycidyl etherobtained from bisphenol A, diglycidyl ether obtained fromtetramethyldihydroxybiphenyl, glycidyl ethers obtained from phenolicnovolak resins and cresylic novolak resins, and halogenation products ofthese compounds. Among them, diglycidyl ether obtained from bisphenol Ais preferred not only because the diglycidyl ether can be used invarious application fields in the form of a liquid or solid, wherein theform of the diglycidyl ether varies depending on the molecular weightthereof, but also because the diglycidyl ether is generally commerciallyavailable as “bisphenol A type epoxy resin”.

Examples of low molecular weight epoxy compounds having a glycidyl groupbonded to an ester linkage include diglycidyl phthalate, diglycidylmaleate, diglycidyl terephthalate, diglycidyl isophthalate, diglycidylnaphthalenedicarboxylate, diglycidyl biphenyldicarboxylate, diglycidylsuccinate, diglycidyl fumarate, diglycidyl glutarate, diglycidyladipate, diglycidyl suberate, diglycidyl sebacate, diglycidyldecandicarboxylate, diglycidyl cyclohexanedicarboxylate, triglycidyltrimellitate, glycidyl esters obtained from dimmer acids, andhalogenation products and oligomers of these compounds.

Examples of epoxy compounds having a glycidyl group bonded to a nitrogenatom include triglycidyl isocyanurate, tetraglycidyldiaminodiphenylmethane, triglycidyl aminophenol, diglycidyl aniline,diglycidyl toluidine, tetraglycidyl methaxylenediamine, tetraglycidylhexamethylenediamine, tetraglycidyl bisaminomethylcyclohexane, glycidylcompounds obtained from hydantoin compounds, and halogenation productsand oligomers of these compounds.

Examples of alicyclic epoxy compounds includebis(3,4-epoxycyclohexyl)adipate, bis(3,4-epoxycyclohexyl)terephthalate,3,4-epoxycyclohexyl-3,4-epoxycyclohexane carboxylate,3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,bis(3,4-epoxycyclohexyl-methyl)oxalate,bis(3,4-epoxy-6-methylcyclohexyl)adipate,bis(3,4-epoxycyclohexylmethyl)pimelate, and vinylcyclohexene dioxide.

Examples of polymers having an epoxy group include a polymer which hasan epoxy group (such as a glycidyl group) in a terminal, a side chain ora branched chain thereof and which has a weight average molecular weightof from 800 to 5,000,000. Specific examples of epoxy group-havingpolymers include a resin having a polyester skeleton, a resin having apolyamide skeleton and a homopolymer or copolymer of a monomer having apolymerizable unsaturated double bond.

From the viewpoint of using the curable composition of the presentinvention for a paint or the like, the above-mentioned polymer having anepoxy group is preferred, and the above-mentioned homopolymer orcopolymer of a monomer having a polymerizable unsaturated double bond ismore preferred. This polymer is a polymer produced by polymerizing atleast one monomer having an epoxy group or a copolymer produced bycopolymerizing a monomer having an epoxy group and a monomer having noepoxy group.

Examples of monomers having both a polymerizable unsaturated double bondand an epoxy group include glycidyl esters of (meth)acrylic acid andmethylglycidyl esters of (meth)acrylilic acid, such as glycidyl(meth)acrylate and β-methylglycidyl (meth)acrylate; a glycidyl ether andmethylglycidyl ether of an allylalcohol; N-glycidyl acrylate amide; andglycidyl vinylsulfonate. In the present invention, the term“(meth)acrylic acid” means any one of acrylic acid and methacrylic acid.

Examples of monomers which have a polymerizable, unsaturated double bondbut have no epoxy group and which are copolymerizable with theabove-mentioned monomer having an epoxy group include acrylic monomers,such as esters of (meth)acrylic acid and esters of (meth)acrylic acidhaving a hydroxyl group. Monomers other than acrylic monomers can alsobe used. Examples of esters of (meth)acrylic acid include methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate,tert-butyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate,2-ethyloctyl (meth)acrylate, benzyl (meth)acrylate, dodecyl(meth)acrylate, phenyl (meth)acrylate and stearyl (meth)acrylate.Examples of esters of acrylic acid which have a hydroxyl group indude2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and2-hydroxycyclohexyl (meth)acrylate. Examples of monomers (other thanmentioned above) which are copolymerizable with a monomer having anepoxy group include hydrocarbons, such as styrene, α-methylstyrene andvinyltoluene; nitrites, such as acrylonitrile and methacrylonitrile;amides, such as acrylamide, methacylamide and methylolacrylamide;esters, such as dialkyl esters of fumaric acid and dialkyl esters ofitaconic acid; vinyloxazoline; vinyl acetate; vinyl propionate; laurylvinyl ether; vinyl monomers having a halogen atom; and vinyl monomershaving a silicon atom.

In the curable composition of the present invention, as the epoxycompound (a), it is preferred to use a glycidyl group-containing acrylicresin especially in the field of a paint which is used in the outdoor,because a cured composition obtained by the curable composition hasexcellent durability (such as excellent weatherability or excellentabrasion resistance). In the present invention, the term “glycidylgroup-containing acrylic resin” means an acrylic resin having a glycidylgroup. Preferred examples of glycidyl group-containing acrylic resininclude a copolymer comprised mainly of methyl methacrylate and glycidylmethacrylate, and a copolymer comprised mainly of methyl methacrylate,glycidyl methacrylate and styrene.

With respect to a glycidyl group-containing polymer for use in a paint,the number average molecular weight thereof is preferably in the rangeof from 1,000 to 100,000. From the viewpoint of the excellentfilm-forming properties of the paint, the excellent smoothness of a filmobtained from the paint, and the excellent kneadability with the curingagent (b), the number average molecular weight of the glycidylgroup-containing polymer is more preferably from 1,000 to 30,000, stillmore preferably from 1,500 to 20,000.

In the present invention, the number average molecular weight of acompound is measured by gel permeation chromatography (GPC) using acalibration curve obtained with respect to monodisperse standardpolystyrene samples and, hence, is obtained as a value relative to thatof the monodisperse standard polystyrene samples used.

In the present invention, the epoxy equivalent of the epoxy compound (a)is generally in the range of from 85 to 10,000 g/eq; however, it is notrequired that the epoxy equivalent of the epoxy compound (a) be withinthe range. With respect to an epoxy compound comprising a glycidyl etherobtained from bisphenol A, the epoxy equivalent thereof is generally inthe range of from 180 to 5,000 g/eq. On the other hand, with respect toa glycidyl group-containing acrylic resin, the epoxy equivalent thereofis generally in the range of from 200 to 5,000 g/eq, preferably from 300to 2,500 g/eq.

In the curable composition of the present invention, with respect to theamount ratio of the epoxy compound (a) to the curing agent (b), it ispreferred that the ratio is from 0.5 to 3 eq, more advantageously from0.7 to 1.5, in terms of the equivalent ratio of the functional group inthe curing agent (b) (which functional group can be reacted with theepoxy group in the epoxy compound (a)) to the epoxy group in the epoxycompound (a). When the above-mentioned amount ratio falls outside of theabove-mentioned range, a disadvantage is likely to be caused wherein thegelation ratio and mechanical properties of a cured composition obtainedby curing the curable composition of the present invention are lowered.

The curable composition of the present invention is produced by mixingthe curing agent (b) and the epoxy compound (a). With respect to themethod for producing the curable composition, there is no particularlimitation. Examples of methods for producing the curable compositioninclude a method in which a mixture of the curing agent (b) and theepoxy compound (a) is kneaded at room temperature or while heating; anda method in which a mixture of the curing agent (b) and the epoxycompound (a) is dispersed or dissolved in water or an organic solvent,and, optionally, the water or organic solvent is removed from theresultant solution.

In the production of the curable composition of the present invention,it is possible that to the epoxy compound (a) is added, as the curingagent (b), a mixture of the polycarboxylic acid mixture of the presentinvention and another curing agents. It is also possible that to theepoxy compound (a) are individually added the polycarboxylic acidmixture and another curing agent.

If desired, the curable composition of the present invention may haveincorporated therein an additive. Examples of additives include curingaccelerators; reactive diluents; fillers and reinforcing agents;flame-retardants, such as antimony trioxide, bromine compounds andaluminum hydroxide; dyes and pigments; mold release agents and fluidityadjusting agents; plasticizers; antioxidants; ultraviolet absorbers;light stabilizers; anti-foaming agents; leveling agents; colorants;titanium dioxide; and solvents. With respect to the amount of theadditive, there is no particular limitation so long as the effect of thepresent invention is not impaired. With respect to the method forincorporating an additive to the curable composition, there is noparticular limitation and a conventional blending method can beemployed.

Examples of curing accelerators include imidazoles, such as2-ethyl-4-methyl imidazole, 2-methyl imidazole and 1-benzyl-2-methylimidazole; tertiary amines, such as dimethylcyclohexylamine,benzyldimethylamine and tris(diaminomethyl)phenol; diazabicyclo alkenesand salts thereof, such as 1,8-diazabicyclo(5,4,0)undec-7-ene;organometal compounds, such as zinc octylate, tin octylate andaluminum/acetyl acetone complex; organic phosphorus compounds, such astriphenyl phosphine and triphenyl phosphite; boron compounds, such asboron trifluoride, boron trifluoride/diethyl ether complex, borontrifluoride/piperidine complex and triphenyl borate; metal halides, suchas zinc chloride and stannic chloride; quaternary ammonium compounds;and alkali metal alcoholates, such as2,4-dihydroxy-3-hydroxy-methylpentane sodium alcoholate; and phenoliccompounds, such as anacardic acid and salts thereof, cardol, cardanol,phenol, nonylphenol and cresol.

Examples of reactive diluents include butyl glycidyl ether, allylglycidyl ether, 2-ethylhexyl glycidyl ether, styrene oxide, phenylglycidyl ether, cresyl glycidyl ether, p-sec-butylphenyl glycidyl ether,glycidyl methacrylate and glycidyl tert-carboxylate.

Examples of fillers and reinforcing agents include coal tar, bitumen,woven cloth, glass fiber, asbestos fiber, boron fiber, carbon fiber,aromatic polyamide fiber, mineral silicate, mica, quartz powder,aluminum hydroxide, bentonite, kaolin, silica aerogel, and metal powder(such as aluminum powder or iron powder).

Examples of mold release agents and fluidity adjusting agents includesilicone, aerosyl, colloidal aluminum silicate containing water, wax,stearates, calcium carbonate and talc.

Examples of plasticizers include pine oil, fluid polymer having lowviscosity, rubbery materials, tar, polysulfide, urethane prepolymer,polyol, diethyl phthalate, dibutyl phthalate, polymers ofepichlorhydrine, dioctyl phthalate, dioctyl adipate and tricresylphosphate.

Examples of ultraviolet absorbers include Tinuvin (trade name;manufactured and sold by Ciba Specialty Chemicals, Switzerland).Examples of steric hindrance amine type light stabilizers includeTinuvin 144 (trade name; manufactured and sold by Ciba SpecialtyChemicals, Switzerland). Examples of phenol type antioxidants includeIRGANOX 1010 and IRGAFOS P-EPQ (trade names; manufactured and sold byCiba Specialty Chemicals, Switzerland). With respect to the method forblending the above-mentioned additive, there is no particular limitationand a blending conventional method can be used.

Examples of pigments include azo pigments; copper phthalocyaninepigments; basic lakes and acid lakes for in-mold decorating; pigmentsfor mordant dye; pigments for construction dye; quinacridone pigments;dioxazine pigments; color pigments, such as carbon black, a chromic acidsalt, ferrocyanides, titanium oxide, selenium sulfides, a silicic acidsalt, a carbonic acid salt, a phosphoric acid salt and metal powder; andextender pigments, such as barium sulfate, barium carbonate, gypsum,alumina white, clay, silica, talc, calcium silicate and magnesiumcarbonate.

Examples of additives (other than mentioned above) include dryers, suchas cobalt naphthenate; anti-skinning agents, such as methoxyphenol andcyclohexanone oxime; thickening agents, such as high polymeric linseedoil, organic bentonite and silica; anti-boiling agents, such as benzoin;and flow modifiers.

The curable composition of the present invention can be cured by heatingor ultraviolet radiation. For example, when the curing of the curablecomposition is performed by heating, the curing temperature is generallyin the range of from room temperature to 250° C., preferably from 80 to200° C., more preferably from 80 to 150° C. The curing time variesdepending the formulation the curable composition; however, the curingtime is generally from several seconds to 200 hours.

The curable composition of the present invention can be advantageouslyused for a paint, an electricity insulating material, an adhesive, amatrix resin for a composite material, and a sealant. In the case of anelectricity insulating material, the composition can be used, forexample, as a material for cast molding, a sealing agent for asemiconductor, an insulating coating and a lamination sheet.

The curable composition of the present invention can be advantageouslyused especially for a coating, such as a powder coating, awater-dispersed slurry coating, an aqueous coating or a solvent typecoating. When a glycidyl group-containing acrylic resin is used as theepoxy compound (a), the curable composition has high curing rate andexcellent durability, such as weatherability. When such a curablecomposition contains no pigment, the curable composition can beadvantageously used for a clear coating, such as a coating for a topcoat for a vending machine, a road material or an automobile.

A coating obtained from the curable composition of the present inventionhas excellent durability (such as weatherability) and can beadvantageously used (as a protective coating for a metal, a concreteprecursor, a wood and a plastic) for a can, an automobile, a ship and aconstruction material.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples and Comparative Examples, whichshould not be construed as limiting the scope of the present invention.

In the following Examples and Comparative Examples, various measurementsand analyses were performed by the following methods.

1. Methods for Analyzing the Characteristics of a Nitrile Mixture

The composition of a nitrile mixture is determined by gas chromatography(GC) or gel permeation chromatography (GPC). The content of thepolynitrile compound (i.e., nitrile compound containing 4 or more cyanogoups) in a trinitrile mixture is measured by GPC. The color tone of thenitrile mixture is measured by UV spectrophotometry using a transmissionmethod. The measurements are performed using the following apparatusunder the following conditions.

[Gas Chromatography]

0.5 g of the nitrile mixture is dissolved in 1.5 g of acetone. Withrespect to the resultant solution, gas chromatography is performed underthe following conditions.

-   -   Apparatus: GC-14 B (trade name; manufactured and sold by        Shimadzu Corporation, Japan)    -   Column: capillary column TC-1 (trade name;

manufactured and sold by GL Science Inc., Japan) (inner diameter: 0.25mm, column length: 30 m)

-   -   Carrier gas: helium    -   Detector: hydrogen flame ionization detector (FID)    -   Column temperature: the column temperature is elevated from        120° C. to 200° C. at an elevation rate of 20° C./min,        maintained at 200° C. for 5 minutes, elevated 200° C. to 250° C.        at an elevation rate of 10° C./min, and maintained at 250° C.        for 10 minutes.    -   Solvent for the sample: acetone

[Gel Permeation Chromatography (GPC)]

2.0 mg of the nitrile mixture is dissolved in 2.0 g of tetrahydrofuran.The resultant solution is subjected to filtration using a filter havinga mesh size of 0.5 μm to thereby obtain a sample solution. With respectto the thus obtained sample solution, GPC is performed under thefollowing conditions.

-   -   Apparatus: HLC-8120GPC (trade name; manufactured and sold by        Tosoh Corporation, Japan)    -   Detector: refractive index detector (RI)    -   Developing solution: tetrahydrofuran    -   Flow rate of developing solution: 1.0 ml/min    -   Column: TSKgel™ GMH_(HR)—N and G1000H_(XL) (trade name; each        manufactured and sold by Tosoh Corporation, Japan)        -   One GMH_(HR)—N column and two G1000H_(XL) columns are            connected in series.    -   Column temperature: 40° C.

[Measurement of the Color Tone]

0.400 g of the nitrile mixture is dissolved in 4.0 ml of diethyleneglycol ether, wherein the volume of the diethylene glycol ether isdetermined by a transfer pipette, to thereby obtain a sample solution.With respect to the obtained sample solution, UV spectrophotometry isperformed in accordance with a transmission method under thebelow-described conditions to thereby obtain the X, Y and Z tristimulusvalues of the sample. From the obtained X, Y and Z tristimulus values,the psychometric lightness L-value, the psychometric chroma a-value andthe psychometric chroma b-value are calculated by the Hunter's colordifference equation.

-   -   Apparatus: UV2500PC (trade name; manufactured and sold by        Shimadzu Corporation, Japan)    -   Sample cell: made of quartz; outer size: 12.4 mm×12.4 mm×45 mm;        optical path: 10.0 mm    -   Reagent: diethylene glycol dimethyl ether (manufactured and sold        by Wako Pure Chemical Industries, Ltd., Japan; reagent of        special grade)    -   Temperature for measurement: 25±2° C.    -   Range of wave length for measurement: from 380 to 780 nm    -   Change rate of wave length for measurement: low rate range        (about 140 nm/min)

2. Methods for Analyzing the Characteristics of a Polycarboxylic AcidMixture

The contents of 1,3,6-hexatricarboxylic acid,3-carboxymethyl-1,5-dicarboxypentane and adipic acid in a polycarboxylicacid mixture are measured by high performance liquid chromatography(hereinafter, frequently referred to as “LC”). The color tone of thepolycarboxylic acid mixture is measured by UV spectrophotometry inaccordance with a transmission method. The nitrogen content of thepolycarboxylic acid mixture is calculated from the nitrogenconcentration of a gas which is generated when the polycarboxylic acidmixture is burned. The above-mentioned measurements are performed bymeans of the below-described apparatuses under the below-describedconditions. With respect to the content of sodium (Na) in thepolycarboxylic acid mixture, the measurement thereof is performed by ionchromatography (Na is derived from sodium hydroxide used for thehydrolysis of the nitrile mixture). [LC Measurement]

In the case where the polycarboxylic acid mixture is present in the formof a solid, the LC measurement is performed as follows. 0.050 g of thepolycarboxylic acid mixture and 0.015 g of isobutylic acid (as aninternal standard) are dissolved in 2.00 g of a developing solutiondescribed below to thereby obtain a sample solution. With respect to theobtained sample solution, LC measurement is performed under thebelow-mentioned conditions. In the case where the polycarboxylic acidmixture is present in the form of a solution thereof, the LC measurementis performed as follows. A certain amount of the polycarboxylic acidmixture (wherein the amount of the polycarboxylic acid mixture is 0.25 gwhen the concentration of the polycarboxylic acid mixture in thesolution is about 20% by weight), and 0.015 g of isobutylic acid (as aninternal standard) are dissolved in 2.0 g of a developing solutiondescribed below to thereby obtain a sample solution. With respect to theobtained sample solution, LC measurement is performed under thebelow-mentioned conditions.

Before the above-mentioned sample solution is subjected to LC, 0.015 gof isobutylic acid (as an internal standard), 0.015 g of1,3,6-hexanetricarboxylic acid and certain amounts of adipic acid areaccurately weighed and dissolved in 2.0 g of distilled water to therebyobtain a solution. This solution is subjected to LC to obtain the peakstrength ratio of 1,3,6-hexanetricarboxylic acid to isobutylic acid, andthe peak strength ratio of adipic acid to isobutylic acid. Using theobtained peak strength ratios, the contents (% by weight) of1,3,6-hexanetricarboxylic acid and adipic acid in the polycarboxylicacid mixture are calculated. With respect to3-carboxymethyl-1,5-dicarboxypentane (which is derived from3-cyanomethyl-1,5-dicyanopentane), the content of this compound in thepolycarboxylic acid mixture is calculated on the assumption that thepeak strength ratio of 3-carboxymethyl-1,5-dicarboxypentane toisobutylic acid is same as that of 1,3,6-hexanetricarboxylic acid toisobutylic acid.

-   -   Apparatus:    -   Column: FinePack SIL C-18S (trade name; manufactured and sold by        JASCO Corporation, Japan)        -   inner diameter: 4.6 mm        -   column length: 150 mm    -   Detector: SPD-6A (trade name; manufactured and sold by Shimadzu        Corporation, Japan)    -   Developing solution: a mixture of acetonitrile (CH₃CN),        distilled water and a 85% by weight aqueous phosphoric acid        solution, wherein the acetonitrile/distilled water/aqueous        phosphoric acid solution weight ratio is 10/990/4    -   Column temperature: 40° C.    -   Flow rate of developing solution: 1.2 ml/min    -   Sample volume: 25 μl

[Measurement of the Color Tone]

0.400 g of a polycarboxylic acid mixture is dissolved in 4.0 ml ofdistilled water, wherein the volume of the distilled water is determinedby a transfer pipette, to thereby obtain a sample solution. With respectto the obtained sample solution, UV spectrophotometry is performed inaccordance with a transmission method under the below-describedconditions to thereby obtain the X, Y and Z tristimulus values of thesample. From the obtained X, Y and Z tristimulus values, thepsychometric lightness L-value, the psychometric chroma a-value and thepsychometric chroma b-value are calculated by the Hunter's colordifference equation.

-   -   Apparatus: UV2500PC (trade name; manufactured and sold by        Shimadzu Corporation, Japan)    -   Sample cell: made of quartz; outer size: 12.4 mm×12.4 mm×45 mm;        optical path: 10.0 mm    -   Reagent: distilled water (manufactured and sold by Wako Pure        Chemical Industries, Ltd., Japan)    -   Temperature for measurement: 25±2° C.    -   Range of wave length for measurement: from 380 to 780 nm    -   Change rate of wave length for measurement: low rate range        (about 140 nm/min)

[Measurement of Nitrogen Content]

A solid containing a polycarboxylic acid mixture is placed on a sampleboat made of quartz and burned to thereby obtain a gas. With respect tothe obtained gas, the nitrogen content thereof is measured under thebelow-described conditions. For the measurement, a calibration curve isobtained with respect to an aqueous solution containing 392 ppm ofpropionitrile (which solution contains 100 ppm of nitrogen) and anaqueous solution containing 3,920 ppm of propionitrile (which solutioncontains 1,000 ppm of nitrogen). The concentration of the polycarboxylicacid mixture in the solid is adjusted so that the range of the nitrogenconcentration of the gas falls within the range (from 100 to 392 ppm) ofthe nitrogen concentration in the above-mentioned calibration curve.

-   -   Apparatus: Total nitrogen analyzer TN-10 (trade name;        manufactured and sold by Mitsubishi Chemical Industries, Ltd.,        Japan)    -   Carrier gas: argon (Ar)    -   Condition for burning: the sample is burned first at 600° C. for        10 seconds and then at 800° C. for 30 seconds.

[Measurement of Sodium Content]

The polycarboxylic acid mixture is dissolved in distilled water toobtain, as a sample solution, an aqueous solution of the polycarboxylicacid mixture. With respect to the obtained aqueous solution, the sodiumcontent thereof is measured by ion chromatography using a column filledwith a cation exchange resin under the below-described conditions. Forthe measurement, a calibration curve is obtained with respect to twoaqueous sodium hydroxide solutions having different sodium hydroxideconcentrations. Using the obtained calibration curve, the sodium contentof the aqueous solution of the polycarboxylic acid mixture iscalculated.

-   -   Apparatus: 8020 series (trade name; manufactured and sold by        Tosoh Corporation, Japan)    -   Detector: conductivity meter CM-8020 (trade name; manufactured        and sold by Tosoh Corporation., Japan)    -   Developing solution: 2 mmol/l aqueous nitric acid solution    -   Column: IC-Cation (trade name; manufactured and sold by Tosoh        Corporation, Japan)    -   Column temperature: 40° C.    -   Flow rate of developing solution: 0.5 ml/min    -   Sample volume: 10 μl

EXAMPLE 1

[Production of Nitrile Mixture]

Using a single electrolytic cell, an electrodimerization reaction ofacrylonitrile was performed as follows.

In the single electrolytic cell, the cathode was comprised of a leadalloy having a conductive surface area of 1 cm×90 cm, and the anode wascomprised of carbon steel having the same conductive surface area asthat of the lead alloy. The anode and cathode were held at a distance of2 mm. As an electrolytic liquid, there was used an emulsion composed of10 parts by weight of an oil phase and 90 parts by weight of an aqueousphase. The aqueous phase was comprised of an aqueous solution having thefollowing composition: about 2.0% by weight of acrylonitrile, about 10%by weight of K₂HPO₄, about 3% by weight of K₂B₄O₇, 0.3% by weight ofethyltributylammonium ethyl sulfate, 0.3% by weight of adiponitrile,0.1% by weight of propionitrile and 0.05% by weight of1,3,6-tricyanohexane. The pH value of the aqueous solution had beenadjusted to about 8 using phosphoric acid. In the emulsion, adissolution equilibrium had been achieved between the aqueous phase andthe oil phase. The oil phase contained about 28% by weight ofacrylonitrile and about 62% by weight of adiponitrile.

The above-mentioned emulsion as the electrolytic liquid wascirculation-supplied to the single electrolytic cell so that the linearvelocity of the emulsion became 1 m/sec at the electrolysis surface, andan electrolysis was performed at an electric current density of 20 A/dm²at 50° C. Simultaneously with the start of the electrolysis, the aqueousphase of the emulsion, which emulsion was sent to an oil/water separatorfrom an electrolytic liquid tank, was treated at a rate of 6 cc/Ah with200 cc of a K⁺ form of an immino-diacetato chelate resin (trade name:Lewatit TP207; manufactured and sold by Bayer AG, Germany) maintained atabout 50° C., and the treated aqueous phase was circulated to theelectrolytic liquid tank.

At the same time, the oil phase was continuously withdrawn from theelectrolytic cell and then from the oil/water separator, and freshacrylonitrile and fresh water were supplied to the electrolytic cell sothat the electrolytic liquid maintained the above-mentioned composition.Ethyltributylammonium ethyl sulfate contained in the aqueous phase wasdissolved in the oil phase and withdrawn from the electrolytic celltogether with the oil phase. Therefore, for compensating for this lossof ethyltributylammonium ethyl sulfate, fresh ethyltributylammoniumethyl sulfate was occasionally supplied to the electrolytic cell.

The above-mentioned electrolysis was performed for 2,000 hours. In theelectrolysis, the electrolysis voltage was initially 3.9, which wasmaintained stably at 3.9 V. The gas generated by the electrolysis had ahydrogen content of 0.16% by volume at completion of the electrolysis.The corrosion rates of the cathode and anode were 0.21 mg/Ah and 0.23mg/Ah, respectively. No heterogeneous corrosion deposit was observed atthe anode. The yields of adiponitrile and 1,3,6-tricyanohexane, relativeto the consumed acrylonitrile, were 90% and 7.5%, respectively.

Subsequently, the withdrawn masses of oil phase were collected andsubjected to extraction with water. From the extract were distilled offacrylonitrile, propionitrile and water, followed by distillation underreduced pressure to remove adiponitrile, thereby obtaining a liquid as adistillation residue. The obtained distillation residual liquid stillcontained 11.5% by weight of adiponitrile.

For removing the adiponitrile from the distillation residual liquid, theliquid was subjected to a batchwise distillation using a distillationcolumn (inner diameter: 32 mm; number of plates: 5) equipped with avacuum jacket under conditions wherein the degree of vacuum was 2.0 mmHgand the column top temperature was in the range of from 120 to 210° C.By the distillation, a fraction comprised mainly of adiponitrile wasremoved, and a distillation residue (A) was obtained.

The obtained distillation residue (A) was a nitrile mixture (B)comprised mainly of 1,3,6-tricyanohexane and contained 4.0% by weight ofadiponitrile, 84.5% by weight of 1,3,6-tricyanohexane, 5.0% by weight of3-cyanomethyl-1,5-dicyanopentane, and 6.5% by weight of a polynitrilecompound.

For separating a nitrile mixture (having a 1,3,6-tricyanohexane contenthigher than that of the nitrile mixture (B)) from the distillationresidue (A), the distillation residue (A) was subjected to a moleculardistillation using a Smith type laboratory molecular distillationapparatus (type 2; manufactured and sold by Shinko Pfaudler Co., Ltd.,Japan; electroheating area: 0.032 m²; made of glass) under conditionswherein the degree of vacuum was 0.1 mmHg, the outer wall heatingtemperature was 180° C., and the supplying rate of the distillationresidue (A) was 2 g/min. The amount of the distillation residue (A) fedto the apparatus was 2,000 g. By the distillation, a distillate and adistillation residue were obtained in amounts of 1,150 g and 850 g,respectively. The distillate was a yellow nitrile mixture (C) comprisedof 93.3% by weight of 1,3,6-tricyanohexane, 5.8% by weight of3-cyanomethyl-1,5-dicyanopentane and 0.9% by weight of adiponitrile.

The color tone of the above-obtained nitrile mixture (C) was measured.As a result, it was found that nitrile mixture (C) had a psychometriclightness L-value of 98.2, a psychometric chroma a-value of −1.18 and apsychometric chroma b-value of 3.68.

The above-mentioned operation was repeated two times (that is, theoperation for obtaining the nitrile mixture (C) was performed threetimes in total), thereby obtaining 1,320 g of the nitrile mixture (C).

In the below-described Examples and Comparative Examples, theabove-mentioned nitrile mixture (C) was used as 1,3,6-tricyanohexane.

[Production of Polycarboxylic Acid Mixture]

161 g of the nitrile mixture (C) and 780 g of a 20% by weight aqueoussodium hydroxide solution (which contained 3.9 mol of sodium hydroxide)were fed to a 1-liter four-necked flask equipped with a reflux condenserand a stirrer. The resultant mixture in the four-necked flask was heatedunder reflux for 24 hours to hydrolyze the nitrile mixture (C), followedby cooling to room temperature, thereby obtaining a hydrolysis reactionmixture (D) in an amount of 916 g.

To 916 g of the hydrolysis reaction mixture (D) was added a 36% byweight hydrochloric acid while cooling the hydrolysis reaction mixture(D) using ice so that the temperature of the hydrolysis reaction mixture(D) did not exceed 20° C., thereby adjusting the pH value of thehydrolysis reaction mixture (D) to 7 to obtain a neutralization reactionmixture (E) in an amount of 1,072 g. Subsequently, 28.2 g of anactivated carbon (trade name: SHIRASAGI; grade: A; manufactured and soldby TAKEDA CHEMICAL INDUSTRIES, LTD., Japan) was added to theneutralization reaction mixture (E), followed by stirring at roomtemperature for 1 hour. Then, the activated carbon was removed from theresultant mixture by filtration, thereby obtaining a transparent,decolored reaction mixture (F) in an amount of 1,065 g.

Subsequently, the pH value of the decolored reaction mixture (F) waslowered to 1 using a 36% by weight hydrochloric acid, thereby convertinga salt of a polycarboxylic acid mixture in the decolored reactionmixture (F) to a polycarboxylic acid mixture. Thus, an aqueous solutioncontaining a polycarboxylic acid mixture was obtained. Then, water wasremoved from the aqueous solution by means of a rotary evaporator atabout 90° C. under reduced pressure, thereby obtaining a solid. Theobtained solid was dried at 40° C. under reduced pressure until thesolid was completely dehydrated, thereby recovering a polycarboxylicacid mixture. The recovered polycarboxylic acid mixture was fed to a10-liter vessel and 4,500 ml of tert-butyl methyl ether was addedthereto, followed by stirring for 1 hour, thereby obtaining apolycarboxylic acid mixture as an extract with the tert-butyl methylether. A salt which had not been dissolved in the tert-butyl methylether was removed by filtration. To the extract (i.e., filtrate) wasadded 50 g of anhydrous magnesium sulfate, followed by stirring at 25°C. for 1 hour, thereby drying the extract. A precipitate was removedfrom the extract by filtration to thereby obtain a filtrate. The solvent(i.e., tert-butyl methyl ether) was distilled off from the obtainedfiltrate at 50° C. under reduced pressure by means of a rotaryevaporator, thereby obtaining a residue. The obtained residue was driedat 40° C. under reduced pressure, thereby obtaining a colorlesspolycarboxylic acid mixture in an amount of 202.7 g. The yield of theactually obtained polycarboxylic acid mixture (hereinafter, this actualyield is referred to simply as a “yield”), relative to the theoreticalyield of the polycarboxylic acid mixture (wherein the theoretical yieldis calculated from the amount of the nitrile mixture (C) fed), was 93%.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 561 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.3% by weight of 1,3,6-hexanetricarboxylic acid, 5.8%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.90, a psychometric chroma a-value of −0.01 and a psychometric b-valueof −0.12. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 100.00, a psychometric chroma a-value of −0.05 and apsychometric chroma b-value of 0.11. The color difference ΔE of thepolycarboxylic acid mixture was 0.23, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.33, a psychometricchroma a-value of −1.21 and a psychometric chroma b-value of 6.87. Thecolor difference ΔE as calculated using the L-value of 97.33, thea-value of −1.21 and the b-value of 6.87 was 7.53.

EXAMPLE 2

1,065 g of a transparent, decolored reaction mixture (F) obtained insubstantially the same manner as in Example 1 was flowed through acolumn packed with 4,000 ml of a styrene type cation exchange resin(trade name: AMBERLYST 15WET; manufactured and sold by Rohm and HaasCo., U.S.A.). The resultant ion-exchanged mixture in the column waswithdrawn from the column by flowing distilled water through the column,thereby obtaining a solution. The obtained solution was filtered bymeans of a TFE filter (pore size: 1 μm) to obtain a filtrate, and thenwater was removed from the obtained filtrate at 90° C. by means of arotary evaporator, thereby obtaining a residue. The obtained residue wascompletely dried at 40° C. under reduced pressure, thereby obtaining200.6 g of a colorless polycarboxylic acid mixture (the yield of thepolycarboxylic acid mixture: 92%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 327 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.4% by weight of 1,3,6-hexanetricarboxylic acid, 5.7%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.86, a psychometric chroma a-value of 0.00 and a psychometric b-valueof 0.25. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.60, a psychometric chroma a-value of −0.15 and apsychometric chroma b-value of 0.60. The color difference ΔE of thepolycarboxylic acid mixture was 0.46, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 98.14, a psychometricchroma a-value of −1.12 and a psychometric chroma b-value of 5.76. Thecolor difference ΔE as calculated using the L-value of 98.14, thea-value of −1.12 and the b-value of 5.76 was 5.51.

EXAMPLE 3

100 g of a polycarboxylic acid mixture obtained in substantially thesame manner as in Example 2 and 100 g of distilled water were fed to a300 ml eggplant type flask. The polycarboxylic acid mixture wasuniformly dissolved in the distilled water at 70° C., thereby obtaininga solution. The obtained solution was allowed to stand to cool thesolution to 25° C. The solution was further allowed to stand at 25° C.to precipitate a crystal. From the start of the precipitation of thecrystal, the temperature of the solution was lowered to 1° C. at a rateof 2° C./hr while appropriately shaking the flask. The crystal wasplaced in a bag made of a filter fabric (200 mesh) and filtered by meansof a centrifugal filter at 3,000 rpm, thereby obtaining 81.3 g of acolorless polycarboxylic acid mixture (the yield of the polycarboxylicacid mixture: 75%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 65 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 99.6% by weight of 1,3,6-hexanetricarboxylic acid, 0.2%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.2% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.61, a psychometric chroma a-value of −0.03 and a psychometric b-valueof 0.02. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.58, a psychometric chroma a-value of 0.03 and apsychometric chroma b-value of 0.21. The color difference ΔE of thepolycarboxylic acid mixture was 0.20, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 99.12, a psychometricchroma a-value of −0.43 and a psychometric chroma b-value of 1.97. Thecolor difference ΔE as calculated using the L-value of 99.12, thea-value of −0.43 and the b-value of 1.97 was 2.05.

EXAMPLE 4

207.1 g of a polycarboxylic acid mixture was obtained (the yield of thepolycarboxylic acid mixture: 95%) in substantially the same manner as inExample 3, except that the pH value of a hydrolysis reaction mixture (D)obtained in substantially the same manner as in Example 1 was adjustedto 13 and the treatment with an activated carbon was performed withrespect to the resultant mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 1,023 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.3% by weight of 1,3,6-hexanetricarboxylic acid, 5.8%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.87, a psychometric chroma a-value of −0.09 and a psychometric b-valueof 0.36. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.79, a psychometric chroma a-value of −0.11 and apsychometric chroma b-value of 0.58. The color difference ΔE of thepolycarboxylic acid mixture was 0.24, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.28, a psychometricchroma a-value of −1.39 and a psychometric chroma b-value of 7.43. Thecolor difference ΔE as calculated using the L-value of 97.28, thea-value of −1.39 and the b-value of 7.43 was 7.64.

EXAMPLE 5

204.5 g of a polycarboxylic acid mixture was obtained (the yield of thepolycarboxylic acid mixture: 94%) in substantially the same manner as inExample 3, except that the pH value of a hydrolysis reaction mixture (D)obtained in substantially the same manner as in Example 1 was adjustedto 12 and the treatment with an activated carbon was performed withrespect to the resultant mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 959 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.4% by weight of 1,3,6-hexanetricarboxylic acid, 5.7%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.80, a psychometric chroma a-value of −0.11 and a psychometric b-valueof 0.29. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.79, a psychometric chroma a-value of −0.09 and apsychometric chroma b-value of 0.50. The color difference ΔE of thepolycarboxylic acid mixture was 0.21, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.22, a psychometricchroma a-value of −1.33 and a psychometric chroma b-value of 7.28. Thecolor difference ΔE as calculated using the L-value of 97.22, thea-value of −1.33 and the b-value of 7.28 was 7.28.

EXAMPLE 6

203.4 g of a polycarboxylic acid mixture was obtained (the yield of thepolycarboxylic acid mixture: 94%) in substantially the same manner as inExample 3, except that the pH value of a hydrolysis reaction mixture (D)obtained in substantially the same manner as in Example 1 was adjustedto 9 and the treatment with an activated carbon was performed withrespect to the resultant mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 832 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.3% by weight of 1,3,6-hexanetricarboxylic acid, 5.9%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.8% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.87, a psychometric chroma a-value of −0.11 and a psychometric b-valueof 0.13. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.90, a psychometric chroma a-value of −0.08 and apsychometric chroma b-value of 0.43. The color difference ΔE of thepolycarboxylic acid mixture was 0.31, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.14, a psychometricchroma a-value of −1.27 and a psychometric chroma b-value of 7.03. Thecolor difference ΔE as calculated using the L-value of 97.14, thea-value of −1.27 and the b-value of 7.03 was 7.51.

EXAMPLE 7

201.2 g of a polycarboxylic acid mixture was obtained (the yield of thepolycarboxylic acid mixture: 93%) in substantially the same manner as inExample 3, except that the pH value of a hydrolysis reaction mixture (D)obtained in substantially the same manner as in Example 1 was adjustedto 5 and the treatment with an activated carbon was performed withrespect to the resultant mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 684 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.2% by weight of 1,3,6-hexanetricarboxylic acid, 5.8%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 1.0% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.96, a psychometric chroma a-value of −0.04 and a psychometric b-valueof 0.00. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.88, a psychometric chroma a-value of −0.07 and apsychometric chroma b-value of 0.26. The color difference ΔE of thepolycarboxylic acid mixture was 0.27, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.29, a psychometricchroma a-value of −1.26 and a psychometric chroma b-value of 6.99. Thecolor difference ΔE as calculated using the L-value of 97.29, thea-value of −1.26 and the b-value of 6.99 was 7.58.

EXAMPLE 8

198.6 g of a polycarboxylic acid mixture was obtained (the yield of thepolycarboxylic acid mixture: 91%) in substantially the same manner as inExample 3, except that the pH value of a hydrolysis reaction mixture (D)obtained in substantially the same manner as in Example 1 was adjustedto 3 and the treatment with an activated carbon was performed withrespect to the resultant mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 1,049 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.3% by weight of 1,3,6-hexanetricarboxylic acid, 5.8%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.87, a psychometric chroma a-value of −0.09 and a psychometric b-valueof 0.18. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.91, a psychometric chroma a-value of −0.12 and apsychometric chroma b-value of 0.47. The color difference ΔE of thepolycarboxylic acid mixture was 0.30, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 96.93, a psychometricchroma a-value of −1.41 and a psychometric chroma b-value of 7.93. Thecolor difference ΔE as calculated using the L-value of 96.93, thea-value of −1.41 and the b-value of 7.93 was 8.39.

EXAMPLE 9

A hydrolysis reaction mixture (D) obtained in substantially the samemanner as in Example 1 was subjected to electrodialysis for removingsodium (Na) using an electrodialyzer (trade name: TS2B-2-10 type;manufactured and sold by TOKUYAMA Corp., Japan) as follows. In theelectrodialyzer, bipolar membranes (trade name: NEOSEPTA BP-1;manufactured and sold by TOKUYAMA Corp., Japan) and K membranes (cationexchange membrane) were alternately arranged at an interval of 0.75 mmbetween an anode and a cathode (wherein a K membrane was disposed atboth ends) to thereby form ten desalting compartments and ten alkalicompartments, wherein each compartment was partitioned by one of thebipolar membranes and one of the K membranes, and wherein the desaltingcompartments and the alkali compartments were alternately disposedbetween the anode and cathode. The effective areas of the bipolarmembrane and K membrane were each 2 dm² per one compartment. As each ofthe anode and cathode, a nickel electrode was used. The electrolysis wasperformed under conditions wherein the current applied was 27 A and thevoltage applied was about 38 V. A tank for supplying the hydrolysisreaction mixture (D) to each desalting compartment (hereinafter, thistank is referred to as “salt tank”), and a tank for supplying an aqueoussodium hydroxide solution to each alkali compartment (hereinafter, thistank is referred to as “alkali tank”) were provided in the outside ofthe above-mentioned compartments. 2,000 g of the hydrolysis reactionmixture (D), 6 liters of a 0.1 N aqueous sodium hydroxide solution andabout 6 liters of a 1 N aqueous sodium hydroxide solution were fed tothe salt tank, the alkali tank and the electrode compartments (i.e.,anode and cathode compartments), respectively. The electrodialysis wasperformed for 18 minutes while circulating the hydrolysis reactionmixture (D) between the salt tank and each desalting compartment andcirculating the 0.1 N aqueous sodium hydroxide solution between thealkali tank and each alkali compartment, each at a flow rate of 3.2l/min (i.e., 6 cm/sec). By the electrodialysis, there was obtained 1,850g of a neutralized solution (H) having a pH value of 7.0.

The neutralized solution (H) had an electrical conductance of 54.3mS/cm. 45 g of an activated carbon (trade name: SHIRASAGI; grade: A;manufactured and sold by TAKEDA CHEMICAL INDUSTRIES, LTD., Japan) wasadded to the neutralized solution (H), followed by stirring at roomtemperature for 1 hour. Then, the activated carbon was removed from theresultant mixture by filtration, thereby obtaining a transparent,decolored neutralized solution (I) in an amount of 1,815 g. Forperforming another electrodialysis, the decolored neutralized solution(I) was fed to the above-mentioned salt tank. As an alkali solution forthe alkali tank, the same aqueous sodium hydroxide solution as used inthe above-mentioned electrodialysis for obtaining the neutralizedsolution (H) was used. The electrodialysis was performed for 50 minutesunder conditions wherein the current applied was 27 A, the voltageapplied was about 38 V and the flow rate was 3.2 l/min (i.e., 6 cm/sec),thereby obtaining 1,400 g of a colorless, transparent desalted solution(J). The electrical conductance of the desalted solution (J) as measuredat completion of the electrodialysis was 1.15 mS/cm. The Na removingratio, which is defined as the ratio of the Na concentration of thedesalted solution (J) to the Na concentration of the hydrolysis reactionmixture (D) used in the electrodialysis treatment, was 99.7%.

The solution (J) was analyzed by LC (liquid chromatography), and it wasfound that the desalted solution (J) contained 461.2 g of1,3,6-hexanetricarboxylic acid, 27.6 g of3-carboxymethyl-1,5-dicarboxypentane and 4.3 g of adipic acid (the yieldof the polycarboxylic acid mixture: 97%).

1,400 g of the desalted solution (J) was fed to a crystallization vesselequipped with a refrigeration medium circulating jacket. The internaltemperature of the vessel was lowered to −3° C. while stirring thecontents of the vessel by means of a Teflon-coated stirring rod,followed by stirring for 24 hours, thereby precipitating a crystal. Theprecipitated crystal was removed by filtration using a PTFE filter (poresize: 1 μm), thereby obtaining a mother liquor. The obtained motherliquor was subjected to a centrifugal separation by means of acentrifugal separator (basket size: 130 φ) at 4,000 rpm for 20 minutes,thereby obtaining a crystal. The obtained crystal had a water content of12%. The crystal was subjected to vacuum drying, thereby obtaining 228 gof a colorless polycarboxylic acid mixture.

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 36 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 99.5% by weight of 1,3,6-hexanetricarboxylic acid, 0.3%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.2% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.97, a psychometric chroma a-value of −0.37 and a psychometric b-valueof 0.01. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.91, a psychometric chroma a-value of −0.29 and apsychometric chroma b-value of 0.07. The color difference ΔE of thepolycarboxylic acid mixture was 0.12, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 99.27, a psychometricchroma a-value of −0.35 and a psychometric chroma b-value of 1.13. Thecolor difference ΔE as calculated using the L-value of 99.27, thea-value of −0.35 and the b-value of 1.13 was 1.32.

EXAMPLE 10

To 914 g of a hydrolysis reaction mixture (D) obtained in substantiallythe same manner as in Example 1 was added 386.5 g of a 36% by weighthydrochloric acid so as to adjust the pH value of the hydrolysisreaction mixture (D) to 1. From the resultant mixture having a pH valueof 1 was removed water by means of a rotary evaporator at a temperatureof from 88 to 92° C. under reduced pressure, thereby obtaining aresidue. The obtained residue was dried at 40° C. under reducedpressure, thereby obtaining a dried residue. The drying of the residuewas performed to such a degree that the change in weight of the residueby drying the residue for 6 hours was not more than 0.01%. Subsequently,the dried residue was fed to a 10-liter vessel and 4,500 ml oftert-butyl methyl ether was added thereto, followed by stirring for 1hour, thereby obtaining a polycarboxylic acid mixture as an extract withthe tert-butyl methyl ether. A salt which had not been dissolved in thetert-butyl methyl ether was removed by filtration. To the extract (i.e.,filtrate) was added 50 g of anhydrous magnesium sulfate, followed bystirring for 1 hour, thereby drying the extract. A precipitate wasremoved from the extract by filtration to thereby obtain a filtrate. Thesolvent (i.e., tert-butyl methyl ether) was distilled off from theobtained filtrate at 50° C. under reduced pressure by means of a rotaryevaporator, thereby obtaining a residue. The obtained residue was driedat 40° C. under reduced pressure, thereby obtaining a polycarboxylicacid mixture in an amount of 213.2 g (the yield of the polycarboxylicacid mixture: 98%).

To 213.2 g of the thus obtained polycarboxylic acid mixture was added303.6 g of distilled water, and the resultant mixture was heated at 45°C. to thereby dissolve the polycarboxylic acid mixture. To the resultantaqueous solution in an amount of 516.8 g was added 336 g of a 35% byweight aqueous sodium hydroxide solution while cooling the aqueoussolution using ice so that the temperature of the resultant aqueousmixture was maintained at a temperature of 25° C. or lower, therebyadjusting the pH value of the aqueous mixture to 7. To the resultantneutralized aqueous mixture was added 10.7 g of an activated carbon(trade name: SHIRASAGI; grade: A; manufactured and sold by TAKEDACHEMICAL INDUSTRIES, LTD., Japan), followed by stirring at roomtemperature for 1 hour. Then, the activated carbon was removed from theresultant aqueous mixture by filtration, thereby obtaining a decoloredaqueous mixture in an amount of 810 g.

To 810 g of the obtained decolored aqueous mixture was added 386 g of a36% by weight hydrochloric acid while cooling the decolored aqueousmixture using ice so that the temperature of the resultant aqueousmixture was maintained at a temperature of 25° C. or lower, therebyadjusting the pH value of the aqueous mixture to 1. From the resultantaqueous mixture having a pH value of 1 was removed water by means of arotary evaporator at a temperature of from 88 to 92° C. under reducedpressure, thereby obtaining a residue. The obtained residue was dried at40° C. under reduced pressure, thereby obtaining a dried residue. Thedrying of the residue was performed to such a degree that the change inweight of the residue by drying the residue for 6 hours was not morethan 0.01%. Subsequently, the dried residue was fed to a 10-liter vesseland 4,500 ml of tert-butyl methyl ether was added thereto, followed bystirring for 1 hour, thereby obtaining a polycarboxylic acid mixture asan extract with the tert-butyl methyl ether. A salt which had not beendissolved in the tert-butyl methyl ether was removed by filtration. Tothe filtrate was added 50 g of anhydrous magnesium sulfate, followed bystirring for 1 hour, thereby drying the extract. A precipitate wasremoved from the extract by filtration to thereby obtain a filtrate. Thesolvent (i.e., tert-butyl methyl ether) was distilled off from theobtained filtrate at 50° C. under reduced pressure by means of a rotaryevaporator, thereby obtaining a residue. The obtained residue was driedat 40° C. under reduced pressure, thereby obtaining a polycarboxylicacid mixture in an amount of 196.7 g (the yield of the polycarboxylicacid mixture: 91%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 480 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.3% by weight of 1,3,6-hexanetricarboxylic acid, 5.8%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic-acid mixture had a psychometric lightness L-value of99.91, a psychometric chroma a-value of −0.02 and a psychometric b-valueof 0.14. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.99, a psychometric chroma a-value of −0.06 and apsychometric chroma b-value of 0.10. The color difference ΔE of thepolycarboxylic acid mixture was 0.23, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.86, a psychometricchroma a-value of −1.17 and a psychometric chroma b-value of 6.34. Thecolor difference ΔE as calculated using the L-value of 97.86, thea-value of −1.17 and the b-value of 6.34 was 6.63.

EXAMPLE 11

161 g of the nitrile mixture (C) obtained in Example 1 and 395.4 g of a36% by weight hydrochloric acid (which contained 3.9 mol of hydrogenchloride) were fed to a 1-liter four-necked flask equipped with a refluxcondenser. The resultant mixture in the four-necked flask was heatedunder reflux for 24 hours to hydrolyze the nitrile mixture (C) whilestirring the contents of the flask by means of a Teflon-coated rotatorwhich was operated by a magnetic stirrer, thereby obtaining a hydrolysisreaction mixture.

The obtained hydrolysis reaction mixture was cooled to room temperatureand allowed to stand for 12 hours. Subsequently, water was removed fromthe hydrolysis reaction mixture by means of a rotary evaporator at atemperature of about 90° C. under reduced pressure, thereby obtaining aresidue. The obtained residue was dried at 40° C. under reducedpressure, thereby obtaining a dried residue. The drying of the residuewas performed to such a degree that the change in weight of the residueby drying the residue for 6 hours was not more than 0.01%. Subsequently,the dried residue was fed to a 10-liter vessel and 4,500 ml oftert-butyl methyl ether was added thereto, followed by stirring for 1hour, thereby obtaining a polycarboxylic acid mixture as an extract withthe tert-butyl methyl ether. A salt which had not been dissolved in thetert-butyl methyl ether was removed by filtration. To the extract (i.e.,filtrate) was added 50 g of anhydrous magnesium sulfate, followed bystirring for 1 hour, thereby drying the extract. A precipitate wasremoved from the extract by filtration to thereby obtain a filtrate. Thesolvent (i.e., tert-butyl methyl ether) was distilled off from theobtained filtrate at 50° C. under reduced pressure by means of a rotaryevaporator, thereby obtaining a residue. The obtained residue was driedat 50° C. under reduced pressure, thereby obtaining a polycarboxylicacid mixture in an amount of 212.1 g (the yield of the polycarboxylicacid mixture: 98%). The drying of the residue was performed to such adegree that the change in weight of the residue by drying the residuefor 6 hours was not more than 0.01%.

To 212.1 g of the thus obtained polycarboxylic acid mixture was added302.4 g of distilled water, and the resultant mixture was heated at 45°C. to thereby dissolve the polycarboxylic acid mixture. To the resultantaqueous solution in an amount of 514.5 g was added 336.0 g of a 35% byweight aqueous sodium hydroxide solution while cooling the aqueoussolution using ice so that the temperature of the resultant aqueousmixture was maintained at a temperature of 25° C. or lower, therebyadjusting the pH value of the aqueous mixture to 7. To the resultantneutralized aqueous mixture was added 10.7 g of an activated carbon(trade name: SHIRASAGI; grade: A; manufactured and sold by TAKEDACHEMICAL INDUSTRIES, LTD., Japan), followed by stirring at roomtemperature for 1 hour. Then, the activated carbon was removed from theresultant aqueous mixture by filtration, thereby obtaining a decoloredaqueous mixture in an amount of 809 g.

To 809 g of the obtained decolored aqueous mixture was added 385 g of a36% by weight hydrochloric acid while cooling the decolored aqueousmixture using ice so that the temperature of the resultant aqueousmixture was maintained at a temperature of 25° C. or lower, therebyadjusting the pH value of the aqueous mixture to 1. From the resultantaqueous mixture having a pH value of 1 was removed water by means of arotary evaporator at a temperature of about 90° C. under reducedpressure, thereby obtaining a residue. The obtained residue was dried at40° C. under reduced pressure, thereby obtaining a dried residue. Thedrying of the residue was performed to such a degree that the change inweight of the residue by drying the residue for 6 hours was not morethan 0.01%. Subsequently, the dried residue was fed to a 10-liter vesseland 4,500 ml-of tert-butyl methyl ether was added thereto, followed bystirring for 1 hour, thereby obtaining a polycarboxylic acid mixture asan extract with the tert-butyl methyl ether. A salt which had not beendissolved in the tert-butyl methyl ether was removed by filtration. Tothe extract (i.e., filtrate) was added 50 g of anhydrous magnesiumsulfate, followed by stirring for 1 hour, thereby drying the extract. Aprecipitate was removed from the extract by filtration to thereby obtaina filtrate. The solvent (i.e., tert-butyl methyl ether) was distilledoff from the obtained filtrate at 50° C. under reduced pressure by meansof a rotary evaporator, thereby obtaining a residue. The obtainedresidue was dried at 40° C. under reduced pressure, thereby obtaining apolycarboxylic acid mixture in an amount of 197.8 g (the yield of thepolycarboxylic acid mixture: 91%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 533 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.2% by weight of 1,3,6-hexanetricarboxylic acid, 5.9%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.92, a psychometric chroma a-value of −0.02 and a psychometric b-valueof −0.13. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 99.87, a psychometric chroma a-value of −0.04 and apsychometric chroma b-value of 0.10. The color difference ΔE of thepolycarboxylic acid mixture was 0.24, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture. Further, anothersample of the polycarboxylic acid mixture was heated at 160° C. for 3hours, and the color tone of the sample of the polycarboxylic acidmixture was measured by the above-mentioned method. As a result, it wasfound that the polycarboxylic acid mixture after heating at 160° C. for3 hours had a psychometric lightness L-value of 97.45, a psychometricchroma a-value of −1.19 and a psychometric chroma b-value of 6.57. Thecolor difference ΔE as calculated using the L-value of 97.45, thea-value of −1.19 and the b-value of 6.57 was 7.00.

Comparative Example 1

916 g of a hydrolysis reaction mixture (D) was obtained in substantiallythe same manner as in Example 1. The pH value of the obtained hydrolysisreaction mixture (D) was adjusted to 1 using a 36% by weighthydrochloric acid while cooling the hydrolysis reaction mixture (D)using ice so that the temperature of the hydrolysis reaction mixture (D)did not exceed 20° C., thereby obtaining 1,371 g of a hydrolysisreaction mixture (K) having a pH value of 1. Subsequently, water wasremoved from the hydrolysis reaction mixture (K) by means of a rotaryevaporator at a temperature of from 88 to 92° C. under reduced pressure,thereby obtaining a residue. The obtained residue was dried at 40° C.under reduced pressure, thereby obtaining a dried residue. The drying ofthe residue was performed to such a degree that the change in weight ofthe residue by drying the residue for 6 hours was not more than 0.01%.Subsequently, the dried residue was fed to a 10-liter vessel and 4,500ml of tert-butyl methyl ether was added thereto, followed by stirringfor 1 hour, thereby obtaining a polycarboxylic acid mixture as anextract with the tert-butyl methyl ether. A salt which had not beendissolved in the tert-butyl methyl ether was removed by filtration. Tothe extract (i.e., filtrate) was added 50 g of anhydrous magnesiumsulfate, followed by stirring for 1 hour, thereby drying the extract. Aprecipitate was removed from the extract by filtration to thereby obtaina filtrate. The solvent (i.e., tert-butyl methyl ether) was distilledoff from the obtained filtrate at 50° C. under reduced pressure by meansof a rotary evaporator, thereby obtaining a residue. The obtainedresidue was dried at 40° C. under reduced pressure, thereby obtaining apolycarboxylic acid mixture in an amount of 203.4 g (the yield of thepolycarboxylic acid mixture: 93%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 4,370 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.5% by weight of 1,3,6-hexanetricarboxylic acid, 5.6%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.2% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of95.23, a psychometric chroma a-value of 0.65 and a psychometric b-valueof 11.71. In addition, for evaluating the thermal stability of thepolycarboxylic acid mixture, a sample of the polycarboxylic acid mixturewas heated at 80° C. for 18 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 80° C. for 18 hours had a psychometric lightnessL-value of 94.79, a psychometric chroma a-value of 0.92 and apsychometric chroma b-value of 12.90. The color difference ΔE of thepolycarboxylic acid mixture was 1.30, wherein the ΔE is an index for thethermal stability of the polycarboxylic acid mixture.

Comparative Example 2

68.88 g of the nitrile mixture (C) obtained in Example 1 and 500 g of a20% by weight aqueous sodium hydroxide solution (which contained 2.5 molof sodium hydroxide) were fed to a 1-liter four-necked flask equippedwith a reflux condenser. The resultant mixture in the four-necked flaskwas heated under reflux for 5 hours while stirring by means of aTeflon-coated rotator which was operated by a magnetic stirrer, therebyperforming the hydrolysis of the nitrile mixture (C). The resultanthydrolysis reaction mixture was cooled. To the hydrolysis reactionmixture was dropwise added 257.0 g of concentrated sulfuric acid (whichcontained 2.67 mol of sulfuric acid) while cooling the hydrolysisreaction mixture using ice so that the temperature of the hydrolysisreaction mixture did not exceed 20° C., thereby obtaining an aqueoussolution having a pH value of 1.

Water was removed from the aqueous solution. The resultant residualsolid was completely dried to thereby obtain a brown solid. The obtainedbrown solid was extracted with ethyl acetate using a Soxhlet'sextractor. From the resultant extract was removed ethyl acetate using arotary evaporator under reduced pressure to thereby obtain 72.5 of ayellow-brown solid polycarboxylic acid mixture (the yield of thepolycarboxylic acid mixture: 77.7%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 1,471 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 93.4% by weight of 1,3,6-hexanetricarboxylic acid, 5.7%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.9% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of95.93, a psychometric chroma a-value of 1.84 and a psychometric chromab-value of 4.48. In addition, for evaluating the thermal stability ofthe polycarboxylic acid mixture, a sample of the polycarboxylic acidmixture was heated at 80° C. for 18 hours, and the color tone of thesample of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture after heating at 80° C. for 18 hours had apsychometric lightness L-value of 94.66, a psychometric chroma a-valueof 1.39 and a psychometric chroma b-value of 9.27. The color differenceΔE of the polycarboxylic acid mixture was 4.98, wherein the ΔE is anindex for the thermal stability of the polycarboxylic acid mixture.

Comparative Example 3

20.0 g of the nitrile mixture (C) obtained in substantially the samemanner as in Example 1 and 130.0 g of a 20% by weight aqueous sodiumhydroxide solution (which contained 0.707 mol of sodium hydroxide) werefed to a 1-liter four-necked flask equipped with a reflux condenser. Theresultant mixture in the fournecked flask was heated under reflux for 5hours while stirring by means of a Teflon-coated rotator which wasoperated by a magnetic stirrer, thereby performing the hydrolysis of thenitrile mixture (C). The resultant hydrolysis reaction mixture wascooled. To the hydrolysis reaction mixture was dropwise added 68.7 g ofconcentrated sulfuric acid (which contained 0.70 mol of sulfuric acid)while cooling the hydrolysis reaction mixture using ice so that thetemperature of the hydrolysis reaction mixture did not exceed 20° C.,thereby obtaining a reaction mixture having a pH value of 1.

The obtained hydrolysis reaction mixture was extracted with 50 ml oftert-butyl methyl ether three times. The resultant extract was driedusing anhydrous magnesium sulfate. The solvent (i.e., tert-butyl methylether) in the resultant dried extract was distilled off to obtain aresidue. The obtained residue was introduced into a mixed solventcomprising 35 ml of acetone and 50 ml of cyclohexane. The resultantmixture was cooled to generate a precipitate in the mixture. The mixturecontaining the precipitate was filtered to recover the precipitate.

The precipitate was subjected to vacuum drying to thereby obtain 19.6 gof a polycarboxylic acid mixture (the yield of the polycarboxylic acidmixture: 72.7%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 1,223 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 97.6% by weight of 1,3,6-hexanetricarboxylic acid, 2.0%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.4% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of96.70, a psychometric chroma a-value of 1.76 and a psychometric chromab-value of 4.07. In addition, for evaluating the thermal stability ofthe polycarboxylic acid mixture, a sample of the polycarboxylic acidmixture was heated at 80° C. for 18 hours, and the color tone of thesample of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture after heating at 80° C. for 18 hours had apsychometric lightness L-value of 95.40, a psychometric chroma a-valueof 1.16 and a psychometric chroma b-value of 8.46. The color differenceΔE of the polycarboxylic acid mixture was 4.62, wherein the ΔE is anindex for the thermal stability of the polycarboxylic acid mixture.

Comparative Example 4

161 g of the nitrile mixture (C) obtained in Example 1 and 395.4 g of a36% by weight hydrochloric acid (which contained 3.9 mol of hydrogenchloride) were fed to a 1-liter four-necked flask equipped with a refluxcondenser. The resultant mixture in the four-necked flask was heatedunder reflux for 24 hours while stirring by means of a Teflon-coatedrotator which was operated by a magnetic stirrer, thereby performing thehydrolysis of the nitrile mixture (C). The resultant hydrolysis reactionmixture was cooled to room temperature and allowed to stand for 12 hoursto generate a precipitate. The precipitate was removed from thehydrolysis reaction mixture by filtration to thereby obtain a filtrate.The obtained filtrate was dried up using a rotary evaporator and furthersubjected to vacuum drying to thereby obtain 220.3 g of a brown solid.To the obtained brown solid was added 881.2 g of distilled water,followed by warming using warm water having a temperature of about 40°C., thereby obtaining an aqueous solution having a pH value of 1.3,wherein the amount of the brown solid dissolved in the distilled waterwas 20% by weight, based on the weight of the aqueous solution. To theaqueous solution was added 22 g of an activated carbon (trade name:SHIRASAGI; grade: A; manufactured and sold by TAKEDA CHEMICALINDUSTRIES, LTD., Japan), followed by stirring at room temperature for 1hour. Then, the activated carbon was removed from the resultant mixtureby filtration, thereby obtaining 1,068 g of a polycarboxylic acidmixture solution (L).

1,068 g of the polycarboxylic acid mixture solution (L) were fed to acrystallization vessel equipped with a refrigeration medium circulatingjacket. The internal temperature of the crystallization vessel waslowered to −3° C. while stirring the contents of the vessel by means ofa Teflon-coated stirring rod, followed by stirring for 24 hours, tothereby precipitate a crystal. The precipitated crystal was removed byfiltration using a PTFE filter (pore size: 1 μm), thereby obtaining amother liquor. The obtained mother liquor was subjected to acentrifugation using a centrifugal separator (basket size: 130 φ) at4,000 rpm for 20 minutes, thereby obtaining a crystal. The obtainedcrystal had a water content of 13%. The crystal was subjected to vacuumdrying, thereby obtaining 89.5 g of a polycarboxylic acid mixture (theyield of the polycarboxylic acid mixture: 41%).

The nitrogen content of the obtained polycarboxylic acid mixture wasmeasured, and it was found that the polycarboxylic acid mixture had anitrogen content of 11,800 ppm by weight. The composition of thepolycarboxylic acid mixture was determined by LC (liquidchromatography), and it was found that the polycarboxylic acid mixturewas comprised of 99.3% by weight of 1,3,6-hexanetricarboxylic acid, 0.5%by weight of 3-carboxymethyl-1,5-dicarboxypentane and 0.2% by weight ofadipic acid.

The color tone of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture had a psychometric lightness L-value of99.08, a psychometric chroma a-value of 0.83 and a psychometric chromab-value of 1.14. In addition, for evaluating the thermal stability ofthe polycarboxylic acid mixture, a sample of the polycarboxylic acidmixture was heated at 80° C. for 18 hours, and the color tone of thesample of the polycarboxylic acid mixture was measured by theabove-mentioned method. As a result, it was found that thepolycarboxylic acid mixture after heating at 80° C. for 18 hours had apsychometric lightness L-value of 96.58, a psychometric chroma a-valueof 1.87 and a psychometric chroma b-value of 7.34. The color differenceΔE of the polycarboxylic acid mixture was 6.77, wherein the ΔE is anindex for the thermal stability of the polycarboxylic acid mixture.Further, for evaluating the thermal stability of the polycarboxylic acidmixture at 160° C., another sample of the polycarboxylic acid mixturewas heated at 160° C. for 3 hours, and the color tone of the sample ofthe polycarboxylic acid mixture was measured by the above-mentionedmethod. As a result, it was found that the polycarboxylic acid mixtureafter heating at 160° C. for 3 hours had a psychometric lightnessL-value of 87.93, a psychometric chroma a-value of 1.37 and apsychometric chroma b-value of 14.74. The color difference ΔE ascalculated using the L-value of 87.93, the a-value of 1.37 and theb-value of 14.74 was 17.61.

Industrial Applicability

The polycarboxylic acid mixture of the present invention has not onlyexcellent color tone (i.e., not discolored and substantially colorlessand transparent) but also excellent color tone stability under heating.Therefore, the polycarboxylic acid mixture of the present invention canbe advantageously used for producing, for example, a paint, a detergent,a builder for a cleaning agent, an anti-limescale agent, a lubricatingoil, and various polycarboxylic acid derivatives, such as esters. By themethod of the present invention, the polycarboxylic acid mixture havingthe above-mentioned excellent properties can be produced in high yield.

1. A polycarboxylic acid mixture comprising 80% by weight or more of1,3,6-hexanetricarboxylic acid, said polycarboxylic acid mixture havinga psychometric lightness L-value of 98 or more, a psychometric chromaa-value of from −2.0 to 2.0 and a psychometric chroma b-value of from−2.0 to 3.0, said polycarboxylic acid mixture having a nitrogen contentof 5,000 ppm by weight or less.
 2. The polycarboxylic acid mixtureaccording to claim 1, which has a psychometric lightness L-value of 99or more, a psychometric chroma a-value of from −1.0 to 1.0 and apsychometric chroma b-value of from −1.0 to 1.0.
 3. The polycarboxylicacid mixture according to claim 1 or 2, which has a nitrogen content of500 ppm by weight or less.
 4. The polycarboxylic acid mixture accordingto claim 1 or 2, which is obtained from a hydrolysis reaction mixtureobtained by hydrolyzing a nitrile mixture comprised mainly of1,3,6-tricyanohexane, said nitrile mixture being obtained as aby-product in a process for producing adiponitrile from acrylonitrile byelectrodimerization or obtained by reacting acrylonitrile withadiponitrile.
 5. A method for producing the polycarboxylic acid mixtureof claim 1, which comprises the steps of: (1) adjusting the pH value ofan aqueous hydrolysis reaction mixture solution obtained by hydrolyzing,in an aqueous medium, a nitrile mixture comprised mainly of1,3,6-tricyanohexane to a level in the range of from 3 to 13, therebyobtaining an aqueous solution containing a salt of a polycarboxylic acidmixture, said nitrile mixture being obtained as a by-product in aprocess for producing adiponitrile from acrylonitrile byelectrodimerization or obtained by reacting acrylonitrile withadiponitrile, (2) treating the aqueous solution with a solid adsorbentto obtain a treated aqueous solution, (3) converting the salt of apolycarboxylic acid mixture in said treated aqueous solution obtained instep (2) to a polycarboxylic acid mixture using an ion exchange resin,an electrodialyzer or an acid, thereby obtaining an aqueous solutioncontaining a polycarboxylic acid mixture, and (4) recovering thepolycarboxylic acid mixture from the aqueous solution obtained in step(3), wherein when the acid is used in step (3) for converting the saltto the polycarboxylic acid mixture, the recovered polycarboxylic acidmixture is subjected to an extraction with an organic solvent for thepolycarboxylic acid mixture to obtain the polycarboxylic acid mixture asan extract with the organic solvent, followed by separation of thepolycarboxylic acid mixture in said extract from the organic solvent. 6.The method according to claim 5, wherein said aqueous medium used forthe hydrolyzing of the nitrile mixture in step (1) is water.
 7. Themethod according to claim 5, wherein, in step (1), the pH value of theaqueous solution is adjusted to a level in the range of from 5 to
 9. 8.The method according to claim 5, wherein said solid adsorbent used instep (2) is at least one adsorbent selected from the group consisting ofan activated carbon, a silica gel and an activated alumina.
 9. Themethod according to claim 5, wherein the conversion of the salt to thepolycarboxylic acid mixture in step (3) is performed using saidelectrodialyzer.
 10. The method according to any one of claims 5 to 9,wherein the aqueous solution obtained in step (3) is subjected tocrystallization before step (4).
 11. A curable composition comprising:(a) a compound having two or more epoxy groups in a molecule thereof,and (b) a curing agent comprising the polycarboxylic acid mixture ofclaim
 1. 12. A paint comprising the curable composition of claim
 11. 13.A cured composition obtained by curing the curable composition of claim11.